Gay Marriage, the U.S. Constitution, and the Evolution of Human Sexuality

This fertilization of a woman's egg by a man's sperm might be seen as showing why the evolution of human sexuality has made the female human nature and the male human nature so different.  But then we must wonder whether gay men and lesbians show this same male/female difference, or whether homosexuality is itself a distinct human nature. And if so, does this mean that homosexual marriages must differ by nature from heterosexual marriages?

The evolution of mammalian reproduction through male insemination of a female and the internal fertilization of the female's egg inside her body means that the minimum parental investment of a mother is usually much greater than the minimum parental investment of a father, which creates a difference on average in the natural mating desires of males and females.  Throughout the evolutionary history of our human ancestors, women had to invest in each offspring at least nine months of pregnancy and many months if not years of nursing and caring for the infant, while men could potentially impregnate many women without any additional investment in the developing fetus or the newborn child.  Men can increase the number of their offspring by promiscuous mating, but women cannot.

If Robert Trivers (1972) is right in his Darwinian theory of mating strategies as determined by parental investment, we should expect that women will typically be more selective than men in choosing mates, and women will typically desire mates with resources that can support her and her children.

David Buss (2016) and others have surveyed the mating preferences of people around the world, and they have found that men prefer to mate with women who are young and physically attractive, while women prefer to mate with men who have economic resources and high social status.  Since the reproductive success of a man depends predominantly on the fertility of his mate, Darwinian theory predicts that the visual cues to fertility in nubile women--such as youth, smooth skin, luscious hair, regular facial features, and good body tone--are sexually attractive to men around the world.  Since the reproductive success of a woman depends predominantly on the ability and willingness of her mate to invest resources in her and her children, we can predict that the social cues to such resources in men--such as wealth, status, older age, and ambition--are sexually attractive to women around the world.

These natural differences in the mating preferences of men and women create conflicts of interests--the battle of the sexes.  Lifelong monogamous marriage is an attempt to strike a truce in that battle by finding a mutually beneficial compromise in which neither sex exploits the other.  A monogamous marriage benefits the wife because her husband agrees to invest emotionally and materially in her and in any children they produce without diverting his resources to other sexually attractive women.  A monogamous marriage benefits the husband because his wife agrees to give him faithful attachment and exclusive sexual access to her, so that he need not suffer from sexual jealousy, and he can be sure that his parental investment is going to his own children and not to children sired by another man.  In this way, her reproductive success becomes his, and vice versa.  For this to work, the man must sacrifice his natural male desire for sexual variety to satisfy his natural desires for conjugal bonding and parental care.  Turning away from the reproductive strategy of maximizing the quantity of his offspring through promiscuous mating, he must follow the reproductive strategy of maximizing the quality of those few offspring who benefit from his intensive paternal care.

If this Darwinian account of the natural sex differences between men and women, and of how heterosexual marriage forces a compromise of these differences, is correct, then we should see confirmation for this in the sex lives of homosexuals.  In The Evolution of Human Sexuality (1979), Donald Symons observed:

"There is no reason to suppose that homosexuals differ systematically from heterosexuals in any way other than sexual object choice . . . . I have argued that male sexuality and female sexuality are fundamentally different, and that sexual relationships between men and women compromise these differences; if so, the sex lives of homosexual men and women--who need not compromise sexually with members of the opposite sex--should provide dramatic insight into male sexuality and female sexuality in their undiluted states.  Homosexuals are the acid test for hypotheses about sex differences in sexuality" (292).

"Homosexual men behave in many ways like heterosexual men, only more so, and lesbians behave like heterosexual women, only more so" (304).

Here and throughout his book, Symons assumed that while heterosexual men and women manifested different human natures as shaped by the evolution of different reproductive strategies,  homosexual men and women did not show any evolutionarily distinct homosexual nature.  But in doing that, he left his readers wondering how homosexuality could have evolved as part of human nature if in fact homosexuality reduces reproductive fitness.  He offered no solution to what I have called the Darwinian puzzle of homosexuality (here).

Symons saw evidence that homosexuals show the natural male-female differences in many respects--including sexual arousal by visual stimuli, the promiscuous seeking of sexual variety, and the importance of physical attractiveness for sexual desirability.  As one indication of this difference, lesbians have no interest in the female nudity displayed in men's magazines like Playboy. And while the male nudity in a magazine like Playgirl is supposed to appeal to heterosexual women, there is reason to believe that the primary audience for Playgirl is homosexual men.

Like heterosexual men, gay males have a natural propensity for seeking out new sexual partners for one night stands.  And for both straight and gay men, purely physical attractiveness is their primary concern.  That's why homosexual men are so careful about their looks--with a emphasis on good grooming, nice clothes, and physical fitness--because they are like heterosexual women in that they can seduce men only if they are physically attractive.  And looking youthful is paramount, which is why gay men are like heterosexual women in their dread of ageing.

Unlike gay men, Symons argued, lesbians are like heterosexual women in that while the physical attractiveness of potential mates is important, at least as important is social intimacy with their mate.  As Symons put it, "among men, sex sometimes results in intimacy; among women, intimacy sometimes results in sex" (301).

Symons suggested: "heterosexual men would be as likely as homosexual men to have sex most often with strangers, to participate in anonymous orgies in public baths, and to stop off in public restrooms for five minutes of fellatio on the way home from work if women were interested in these activities.  But women are not interested" (300).

Symons implied that the institution of heterosexual marriage is an unnatural constraint on male desires imposed by women to satisfy female desires.  Symons is wrong about this, because despite the conflict between male and female desires, there is a fundamental complementarity in their desires for the stable arrangements of marriage and family life.  Even Symons recognized this complementarity in a few passages of his book.  "The desire for sexual variety," he wrote, "dooms most human males to a lifetime of unfulfilled longing; when the desire can be satisfied easily, as among many homosexual men, it often frustrates the satisfaction of other desires, such as those for intimacy and security" (228).

If marriage did not satisfy male desires, it would not be a universal practice of all societies.  If most men found the institution of marriage ultimately unsatisfactory, they would have abolished it long ago.  They have always had the power to do so, because every human society has been ruled by men.

Beginning as early as the 1950s and 1960s in the United States, some homosexuals began arguing for legalizing homosexual marriages as a way of providing social encouragement for homosexual monogamy, although some conceded that monogamous commitment is usually easier for lesbians than for gay men.  And yet many in the gay community have scorned the idea of gay marriage as contrary to the liberationist culture of gay life that should challenge the stultifying bourgeois norms of traditional marriage (Frank 2017).

The first governmental legalization of gay marriage occurred in the Netherlands in 2001, which was followed by some other European countries.  In the United States, Massachusetts was the first state to legalize gay marriage in 2003 through a decision of the Massachusetts Supreme Court (Goodridge v. Department of Public Health).  By the spring of 2015, 37 states allowed gay marriage; and at the end of June of that year, the Supreme Court of the United States declared gay marriage to be a constitutional right in Obergefell v. Hodges.  (I have written about Obergefell in June and July of 2015 and March and April of 2019).

So now, for the first time in human history, we are experimenting with legalized same-sex marriage.  The experiment will determine its success or failure.

Based on my argument for the Darwinian natural law of gay marriage--that it can satisfy the same natural desires for conjugal bonding and parental care that are satisfied in heterosexual marriage--I can predict at least partial success:  some gays (though maybe not most) will marry and sustain long-term monogamous commitments, but this will be more successful among lesbian couples than gay male couples; and many of these same-sex marriages will include parental care of children.  The critical question is what proportion of the gay male couples will manage to constrain the gay male propensity to promiscuous infidelity.

Depending on how one looks at it, we already have one case of a gay male couple that has been legally married for 48 years, and some historians consider them to be the first same-sex couple in history to be legally married.  On May 18, 1970, Michael McConnell and Jack Baker arrived at the Hennepin County Courthouse in Minneapolis, Minnesota, to apply for a marriage license.  Baker was a first-year law student at the University of Minnesota Law School, and he had noticed that the Minnesota statute on marriage licenses did not require that applicants identify their sex.  Having learned in law school the principle that "whatever isn't prohibited by law is permitted," he told McConnell that they had the legal right to marry in Minnesota.  When they arrived at the courthouse, news reporters and photographers were there, because they had alerted the press.  They wanted their filling out an application for a marriage license to be a public statement about the right of gays to marry (McConnell et al. 2016).

Gerald R. Nelson, clerk of district court, Hennepin County, refused to issue the marriage license.  A few months later, Baker filed for a writ of mandamus in Hennepin County District Court, which would have required the officials in Hennepin County to carry out their legal duties in issuing the marriage license.  Among their legal arguments, they claimed that if Minnesota law was interpreted as denying the right of gays to marry, this would violate the United States Constitution by denying them a fundamental right to privacy guaranteed by the Ninth Amendment and by depriving them of liberty and property without due process and of the equal protection of the law, both guaranteed by the Fourteenth Amendment.  In making these arguments, they knew that in the 1970s, a congressional statute said any interpretation of the federal constitution by a state court was entitled to automatic review by the U. S. Supreme Court.  So when the district court judge dismissed their case, they appealed to the Minnesota Supreme Court; and when their constitutional arguments were denied by that court in 1971, they were entitled for review by the U. S. Supreme Court.  This was the case of Baker v. Nelson.

In the Appellant's Jurisdictional Statement, in 1972, the lawyers for Baker and McConnell defended their constitutional right to marry by indicating that same-sex marriage was similar to heterosexual marriage in promoting the two natural ends of marriage--conjugal bonding and parental care--and thus they implicitly made the same natural law argument for same-sex marriage that would be made by Justice Anthony Kennedy in Obergefell v. Hodges in 2015.

To justify a governmental ban on same-sex marriage as consistent with the "equal protection of the laws" guaranteed by the Fourteenth Amendment, the lawyers argued, the Supreme Court's equal protection doctrine would require "a legitimate government interest which is so compelling that no restrictive means can be found to secure that interest, if there is one, than to proscribe single sex marriages" (p. 15).  One possible legitimate interest for marriage law might be to make marriage turn on marriage partners being willing and able to procreate and raise children.  But this purpose cannot justify banning same-sex marriage.  they explained:  "There is nothing in the nature of single sex marriages that precludes procreation and child rearing.  Adoption is quite clearly a socially acceptable form of procreation.  It already renders procreative many marriages between persons of opposite sexes in which the partners are physically or emotionally unable to conceive their own children.  Of late, even single persons have become eligible to be adoptive parents" (pp. 14-15).

Actually, of course, the marriage law in Minnesota and other states validates the marriages of childless heterosexual couples, and thus it recognizes the conjugal bonding of a couple as a natural purpose of marriage that stands independently of parental care as a sufficient ground for marriage.  But then to allow marriage for childless heterosexual couples while denying it for childless same sex couples violates the constitutional standard of "equal protection of the laws."

In support of this conclusion, the lawyers for Baker and McConnell cited the Court's recent decision in Reed v. Reed, 92 S. Ct. 251 (1971), which struck down an Idaho statute, which provided that as between persons equally qualified to administer estates, males must be preferred to females, as an unconstitutional violation of the equal protection clause of the Fourteenth Amendment.  The lawyers quoted from that decision:

"In applying that clause, this Court has consistently recognized that the Fourteenth amendment does not deny to States the power to treat different classes of persons in different ways. [Citations omitted.]  The Equal Protection Clause of that Amendment does, however, deny to States the power to legislate that different treatment be accorded to persons placed by a statute into different classes on the basis of criteria wholly unrelated to the objective of that statute.  A classification 'must be reasonable, not arbitrary, and must rest upon some ground of difference having a fair and substantial relation to the object of the legislation, so that all persons similarly circumstanced shall be treated alike.'  Royster Guano Co. v. Virginia, 253 U.S. 412, 415 (1920)."

Since childless same sex couples are "similarly circumstanced" to childless heterosexual couples, they must be treated alike.  They must have an equal right to marry, because they are equal in their need for marriage to secure their natural desire for conjugal bonding: "how better may two people pledge love and devotion to one another than by marriage" (p. 7).

In their Appellee's Motion to Dismiss Appeal and Brief (Baker v. Nelson [1972], no. 71-1027), the lawyers for Hennepin County made six arguments for dismissing the appeal.  Their first argument was that this case did not present "a substantial federal question," because the power to regulate marriage was exclusively a power of the states, and therefore beyond the authority of the national government.  This turned out to be the decisive argument for the Court, because a law clerk for Justice Harry Blackmun drafted a one-sentence rejection of the appeal that was approved by the Court:  "The appeal is dismissed for want of a substantial federal question" (Baker v. Nelson, 409 U.S. 810 [1972]).  Years later, the man who had been Blackmun's law clerk in 1972 said "I just didn't think the Court was ready at the time to take on the issue" (Bravin 2015).  In subsequent years, this was cited by opponents of same sex marriage as a decisive precedent of the Court allowing states to ban same sex marriage.  Overruling this decision in Obergefell v. Hodges in 2015, Justice Anthony Kennedy wrote: "The Court now holds that same-sex couples may exercise the fundamental right to marry.  No longer may this liberty be denied to them.  Baker v. Nelson must be and now is overruled, and the State laws challenged by Petitioners in these cases are now held invalid to the extent they exclude same-sex couples form civil marriage on the same terms and conditions as opposite-sex couples."

The second argument for dismissing the appeal was that the right to marry was not in fact being withheld from either Baker or McConnell, because each was free to marry as long as they married someone of the opposite sex (p. 5).  But this ignored the fact that the Court had rejected a similar argument a few years earlier in Loving v. Virginia (1967), when Virginia's law against interracial marriage was declared an unconstitutional violation of equal protection, which rejected the specious argument of the lawyers for the state of Virginia that blacks and whites had an equal right to marry as along as they married someone of the same race as they.  And, indeed, the lawyers for Baker and McConnell stressed the importance of the Loving decision in showing how the equal protection doctrine should uphold the constitutional right  not only to interracial marriages but also to same-sex marriages.  To refute this reasoning, one would have to show that while interracial marriages do not subvert the natural purposes of marriage, same-sex marriages do.

This points to the third argument for dismissing the appeal--the claim that same-sex marriage would "defeat and destroy" the institution of marriage (p. 6).  This claim has been developed by many of the opponents of same-sex marriage--such as Maggie Gallagher and Hadley Arkes.  Arkes has said: "It is not marriage that domesticates men; it is women" (1993).  Marriage domesticates men when they marry women, not when they marry other men.  If Symons is right about the socially destructive natural propensity of men to promiscuity, then heterosexual monogamous marriage is good for men and for social order generally because it forces men to constrain their restless desire for sexual variety by compromising with the demands of women for monogamous permanence and fidelity.  If "marriage equality" means that same-sex and opposite-sex unions are treated equally as marriages, then this is based on a lie about human nature--the lie that there is no natural difference between a union of two men or two women and a union of a man and a woman.  In fact, few male same-sex couples adhere to the norm of monogamous fidelity because they are free from the constraints of female sexuality.  Stretching the definition of marriage to include the same-sex marriage of men will therefore destroy the heterosexual marital norm of monogamous fidelity and permanence (Corvino and Gallagher 2012, 129-149).

But as John Corvino and others have noted, this ignores the fact that lesbian couples do seem to have a strong natural propensity to monogamous stability.  If it's good to marry a woman, it must be even better for women to marry other women!  Gallagher concedes this when she cites research showing "that whereas monogamy is correlated with relationship satisfaction for heterosexuals and lesbians--that is, having monogamy is associated with being in a happy relationship--for gay men, there's no association between sexual exclusivity and the satisfaction of the relationship" (Corvino and Gallagher 2012, 135).

So should we legalize heterosexual marriages and lesbian marriages, while banning gay male marriages, because legally recognizing the marriage of men would endorse male promiscuity and thus destroy the traditional norm of monogamy for marriage?  Or should we agree with Corvino that while we must grant that "on average, sexual exclusivity appears less important for gay men's relationship success (longevity, satisfaction, etc.) than it does for that of heterosexuals or lesbians," inferring from this that legalizing gay male marriage would destroy the norm of monogamous marriage is both implausible and unjust?

It is empirically implausible to claim that the traditional norm of life-long monogamous fidelity will be destroyed if we legalize gay male marriages, some of which will not be strictly monogamous.  Given the small number of gay men, and the much larger number of heterosexual men, there are probably many more heterosexual couples who become "swingers" in "open marriages" than there are promiscuous gay men.  And yet even with the large number of heterosexual people who ignore the norm of sexual exclusivity, monogamous fidelity is still for most people a marital norm, if only as an ideal often violated in practice.

It is also unjust to deny gays the right to marry because some gay couples will fail to be monogamous.  This is unjust because many heterosexual married couples choose not to embrace sexual fidelity as a norm for them, and yet they are just as truly married as any other legally married couple.  And, again, it is unjust to use the gay male propensity to promiscuity as a reason to deny the right to marry to lesbian couples who might enforce sexual exclusivity in their marriage, or to those gay male couples who do strive for monogamous fidelity.

Nevertheless, one might question the relevance of such moral arguments to the constitutional standards enforced by the Supreme Court.  And, indeed, the fourth argument of the lawyers in Baker v. Nelson for dismissing the appeal of Baker and McConnell was that there was no clear language in the Constitution of the United States giving homosexuals the right to marry.  The lawyers wrote: "The framers of our Constitution and the statesmen of the States of the Union ratifying the same, would have never conceived that the protections which were afforded the people under this historic document would ever be utilized in such a way as to bring about the result sought by the appellants herein" (p. 8).  This is undoubtedly true--that those who framed and ratified the Fourteenth Amendment never intended or anticipated that the guarantee of "equal protection" would be interpreted to mean that state bans on same-sex marriage were unconstitutional.  But it could also be said that they would never have foreseen that the Fourteenth Amendment would be read as striking down state laws banning interracial marriage as unconstitutional, as was done in Loving v. Virginia, a decision favorably cited by the lawyers arguing against the claims of Baker and McConnell.  Even if the decision in Loving cannot be grounded in the original understanding of those who framed and ratified the Fourteenth Amendment, it can be grounded in the original meaning of the words "equal protection of the laws" in that amendment, because we can see how laws prohibiting interracial marriage deprive interracial couples of equal protection in a way that does not rationally serve any legitimate interest of state government.  Similarly, we can see how laws prohibiting same-sex marriages deprive homosexual couples of equal protection in an arbitrary way that does not have any rational relation to any proper purpose of government.

The fifth argument against the appeal of Baker and McConnell invoked religious belief.  "Our country, and our Constitution, were founded upon basic religious principles and one of the most basic of such principles is that marriage is an institution ordained by God and that such institution is to be entered into by a man and a woman as husband and wife."  The lawyers for the appellees do not explain, however, where exactly those religious principles enter the Constitution.  Nor do they respond to the claim of the lawyers for Baker and McConnell that hostility to homosexuality in the Bible shows an unreasonable prejudice against homosexuals that has no constitutional status.

The final argument against Baker and McConnell's appeal points to one of the most peculiar facets of this case.  The questions raised by this appeal are said to be moot, because while Baker and McConnell claim they have been denied the right to marry, they did in fact marry each other on September 3, 1971, about six weeks before the Minnesota Supreme Court ruled against their appeal!

This marriage was made possible by some clever legal maneuvering by Baker and McConnell.  When they realized that adoption would give them about 90 percent of the benefits of marriage, because the adoptee becomes part of a family, they arranged to have Baker adopted by McConnell.  Also, in the process of adoption, they changed Jack Baker's name to Pat Lyn McConnell, so that the gender free name "Pat Lyn" would make it harder to identify them as two men when they applied for a marriage license.  They then moved for a short time to Mankato, Minnesota, in Blue Earth County, where they applied and received a marriage license for Michael McConnell and Pat Lyn McConnell on August 9, 1971.  They married a month later in Minneapolis.  They could then claim that when the state supreme court later ruled against their Hennepin County license, this would not deny the legality of their wedding.

In their book about their life, published in 2016, six months after the Obergefell ruling, McConnell and Baker tell the story of a marriage that has now lasted for 48 years.  While recognizing that most gay men seek only short-term relationships, they decided that they would be one of those few gay male couples who would make a lifelong commitment to one another.  They admit, however, that they did not see this marital commitment to one another as requiring sexual exclusivity.  They agreed that they would be free to date other men (McConnell et al. 2016, 15, 38-39, 50-51).  Here then they confirmed Symons' point about the promiscuity of male homosexual sexuality.  But they also indicate that as they grew older, the gay bars and bathhouses were less appealing to them, and they settled into a largely monogamous marriage lasting a lifetime.  They thus achieved the first natural end of marriage--conjugal bonding, or what Corvino has called the "mutual lifelong caregiving" function of marriage (Corvino and Gallagher 2012, 14-18).

                                      A Video on the Gay Marriage of McConnell and Baker


They also sought the second natural end of marriage--the parental care of children.  For over two years, they tried to adopt a child.  They decided that they would be open to adopting any child under the age of 6, even one that might need special care.  But when all of their applications at the adoption agencies failed, they finally decided that their marriage would be childless.

Michael McConnell and Jack Baker have not achieved the ideal marriage of a couple with lifelong perfect monogamy and the rearing of children.  But it's hard to believe--as many opponents of same-sex marriage would say--that this is not a "real marriage."


REFERENCES

Appellant's Jurisdictional Statement, Baker v. Nelson, U.S. Supreme Court docket no. 71-1027.

Appellee's Motion to Dismiss Appeal and Brief, Baker v. Nelson, U.S. Supreme Court docket no. 71-1027.

Arkes, Hadley. 1993. "The Closet Straight." National Review, July 5.

Bravin, Jess. 2015. "Supreme Court Clerk Remembers First Same-Sex Marriage Case." The Wall Street Journal, May 1.

Buss, David. 2016. The Evolution of Desire: Strategies of Human Mating. Revised and updated edition. New York: Basic Books.

Corvino, John, and Maggie Gallagher. 2012. Debating Same-Sex Marriage. Oxford: Oxford University Press.

Frank, Nathaniel. 2017. Awakening: How Gays and Lesbians Brought Marriage Equality to America. Cambridge, MA: Harvard University Press.

McConnell, Michael, Jack Baker, and Gail Langer Karwoski. 2016. The Wedding Heard 'Round the World: America's First Gay Marriage. Minneapolis: University of Minnesota Press.

Symons, Donald. 1979. The Evolution of Human Sexuality. Oxford: Oxford University Press.

Trivers, Robert. 1972. "Parental Investment and Sexual Selection." In Bernard Campbell, ed., Sexual Selection and the Descent of Man, 1871-1971, 136-79. Chicago: Aldine.

Edward Westermarck: The First Sociobiologist and Proponent of Darwinian Natural Right

I have often written on this blog about how Edward Westermarck has influenced my thinking about sociobiology and Darwinian natural right--particularly, through his theory of incest avoidance (here, here, here, here, and here).  Now, I am thinking more about this after reading Stephen Sanderson's paper--"Edward Westermarck: The First Sociobiologist"--in the Oxford Handbook of Evolution, Biology, and Society (2018), edited by Rosemary Hopcroft, pp. 63-86.  This paper can be found online.

Sanderson is a sociologist who is one of the few sociologists who has argued for founding sociology on a biological science of human nature.  (I wrote a series of posts on Sanderson's work in April and May of 2016.)  His paper presents Westermarck as the most unfairly neglected sociologist who showed how a Darwinian science of human social behavior could deepen sociology as a true science.  He explains how sociologists today mostly ignore Westermarck, because they have embraced the position of Emile Durkheim--one of Westermarck's opponents--that social life can only be explained through environmentalism and social determinism, which requires a rejection of any Darwinian science of human nature.  This Durkheimian scorn for the biological basis of social life explains why so few sociologists today are open to sociobiology and evolutionary psychology.

In his attempt to revive Westermarck's Darwinian thinking in the social sciences, Sanderson explains how Westermarck's accounts of the evolutionary history of human marriage (including the incest taboo) and of human moral psychology were ahead of his time in anticipating the work of sociobiology and evolutionary psychology over the past 50 years.

In doing this, Sanderson identifies me as one of the scholars developing a Darwinian understanding of human morality in the tradition of Westermarck.  He rightly sees that Westermarck's Darwinian account of morality as rooted in evolved human emotions was a denial of Kant's transcendentalist rationalism in ethics.  Contrary to Kant, reason by itself never moves us to act.  In our mental and moral life, the intellect is how, the emotions why.  I have elaborated that point in various posts (here, here, here, and here).

Sanderson summarizes and apparently accepts my defense of Darwinian natural right as founded on 20 natural desires.  (I have written about Sanderson's Darwinian account of the natural desires here.)

But then he cannot accept my defense of "Darwinian conservatism."  He writes:

"I do not intend to be presenting Arnhart's conservative political philosophy, as represented in the previously presented five principles, as 'correct.' I simply offer it as a leading example of an attempt to ground a moral philosophy in Darwinian theoretical principles.  As one might imagine, nearly all those on the political Left are anti-Darwinian with respect to moral and political philosophy.  There is the occasional exception, however (e.g., Peter Singer's book A Darwinian Left [1999]).  Westermarck himself was a liberal" (note 4).

Sanderson does not mention my critique of Singer's Darwinian Left in Darwinian Conservatism or my general arguments against attempts to enlist Darwinian science in support of socialism (here and here).

In saying that "Westermarck himself was a liberal," Sanderson intimates that he agrees with Antti Lepisto's claim that Westermarck would not have agreed with Darwinian conservatism, because Westermarck often took a liberal or reformist position on marriage and family life, as in his arguing for liberalizing marriage and family law to make divorce easier, and in his generally tolerant attitude towards homosexuality.  But as I have said in response to Lepisto (here), my Darwinian conservatism is a liberal conservatism that embraces the classical liberalism of Locke, Smith, and Hayek; and such classical liberalism can be supported by a Darwinian science of morality and social order.  Westermarck's classical liberalism is suggested by his adoption of Adam Smith's ideas in The Theory of Moral Sentiments.

Gini Coefficients for Inequality in Locke's State of Nature and Lockean Liberal Orders

In various posts (here and here), I have argued that the Darwinian science of human evolutionary history largely confirms John Locke's view of the state of nature as the natural condition of equal liberty, while also confirming (here) that Lockean liberal social orders can approximate that equal liberty that human beings enjoyed in the state of nature.

Over the past 10 years, Darwinian social scientists have brought more precision to the empirical study of the evolutionary history of inequality by finding ways to calculate the Gini coefficients for inequality in human societies over the past 10,000 years; and this new research supports the Lockean liberal account of human history.

The Gini coefficient is a statistical measure of the distribution of income or wealth in a society that ranges from 0 (every individual or household has exactly the same amount) to 1 (one individual or household owns all of the income or wealth, and others have nothing).  (The Wikipedia article on the Gini coefficient is a good survey.)  Gini coefficients below 0.30 are considered indicators of extremely low inequality.  Gini numbers above 0.50 are indicators of extremely high inequality.  So, for example, if the richest 20% have 80% of all income, that would count as an income Gini coefficient of 0.60.  That's the income Gini number for Haiti, which is one of the highest in the world.  The number for the U.S.A is in the middle--0.40.  The lowest numbers are usually for the Nordic social democracies--such as Denmark, Norway, and Finland--which are around 0.24.

Monique Borgerhoff Mulder and her colleagues (2009) have calculated Gini coefficients for 21 historical and contemporary small-scale populations based on ethnographic data.  Timothy Kohler and his colleagues (2017) have calculated Gini coefficients for 63 archaeological sites based on archaeological data.

The 21 societies studied by Borgerhoff Mulder and her coauthors included hunter-gatherer, horticultural, pastoral, and agricultural societies.  They calculated the intergenerational transfer of three kinds of wealth--material (such as household goods, land, and livestock), embodied (such as strength, knowledge, and skills), and relational (such as the individual's position in social networks). They found that material wealth was more easily transmitted across generations, as compared with embodied or relational wealth.  And therefore there was more inequality in pastoral and agricultural societies where material wealth (such as livestock and land) was important, as opposed to foraging and horticultural societies, where material wealth was less important, and consequently there was less inequality.

They estimated Gini coefficients for wealth at 0.25 for hunter-gatherers, 0.27 for horticuluralists, 0.42 for herders, and 0.48 for farmers.  Hunter-gatherers typically have a strong egalitarian ethos, and they found that indeed their level of inequality is much lower than for other societies.  But, still, hunter-gatherers have never achieved absolute equality.  They found that among the foraging societies they examined, a child born into a family in the top 10% of wealth is 3-5 times as likely on average to remain at the top than is a child born into a family in the bottom 10%.

Locke recognized this because while he saw the American Indians in the state of nature as showing relative equality compared with other societies, he also saw that all human societies will have some inequality due to individual differences in age, birth, talents, social networks, and luck, which will make some individuals more highly ranked than others.  So, for example, among hunter-gatherers, some individuals would inevitably distinguish themselves as skillful hunters or as leaders of their groups.  Borgerhoff Mulder and her colleagues confirmed this, and concluded that those Marxist anthropologists who wanted to find "primitive communism" in foraging societies were mistaken (Smith et al. 2010, 31).

Nevertheless, the reputation of hunter-gatherers for being egalitarian is warranted when they are compared with herding and farming societies that have much higher Gini numbers.  And yet, among modern nation-states, those that have Lockean liberal social orders have lower inequality.  The United States has a slightly lower Gini number than the small-scale herding and farming societies.  And the Nordic capitalist welfare states have Gini numbers about the same as the foraging societies.  (I have written about the Nordic social democracies as liberal regimes here.)  This suggests that modern Lockean liberal social orders approximate the equal liberty of human beings in the state of nature.

There are, however, some difficulties in the research of Borgerhoff et al. that have been pointed out by critics.  First, their sample of foraging societies does not include those high-density sedentary coastal foragers with Chiefdoms--such as the Northwest Coast Indians, the Chumash, and the Calusa--who were known to have hereditary nobility and even slavery.

The second difficulty is that their ethnographic sample of foraging societies includes those like the Ache of Paraguay and the Hadza of Tanzania whose foraging life has been altered by contact with modern herding and farming societies.  For that reason, it might not be clear that these foraging societies are really representative of our original evolutionary ancestors (Caldararo 2011; Kelly 2010).  One can respond to this difficulty by pointing out that the Ache and the Hadza have not been completely assimilated into modern societies, and that ethnographers have been able to compile a record of their history before contact (Smith et al. 2011; Hill and Hurtado 1996; Marlowe 2010).  While it is true that people like the Ache and the Hadza are not "living fossils" who have been "frozen in time," it is also true that they at least resemble the societies of our ancient foraging ancestors.

Instead of relying on ethnographic studies of inequality in historical and contemporary populations, Kohler and his colleagues (2017) have gathered archaeological evidence from a sample of 63 sites or groups of sites on four continents dating from around 11,000 to about 2,000 years ago.  For their evidence of inequality, they use house size as a proxy for household wealth.  Measuring the relative sizes of houses as distributed over an archaeological site, they can measure the inequality manifested in the relative wealth of households with large houses versus households with smaller houses (Kohler and Smith 2018).

                             An excavated house at the El Palmillo archaeological site, Mexico

As expected, they found that hunter-gatherers had the lowest Gini coefficients (median = 0.17), the horticulturalists had higher numbers (median = 0.27), and farmers had the highest (median = 0.35).  They were surprised, however, to see that the wealth inequality tended to be higher in the Eurasian sites than in North America and Mesoamerica.  To explain this, they suggested that Eurasia had a greater availability of large mammals that could become domesticated draft animals that could be used for increasing agricultural production and extending the area of land in cultivation, which could have created greater concentrations of wealth in those who controlled this production.

Another possible explanation is that their sample of sites in the New World is slanted towards more egalitarian societies.  For example, they did not include any sites in South America, which could have included the Incan empire centered in Peru, which was probably one of the most unequal agrarian societies, in which llamas and alpacas were domesticated.

And while their Mesoamerican sample included the Mayan city of Tikal with a remarkably high Gini number of 0.62, it also included some cities with surprisingly low Gini numbers, such as Teotihuacan with a Gini of 0.12--lower even than that for the hunter-gatherers!  The archeology of Teotihuacan (AD 400-500) is hard to interpret.  It shows signs of autocratic rule--with its grand avenue and pyramids--but it also shows some signs of broad sharing of power--with a grid of roads, no very large royal palace, and many medium-sized houses.  Thus, pre-Hispanic Mesoamerica seems to exhibit two very different strategies for structuring power--either concentrating and centralizing power in the hands of a few kings and nobles or allowing for a collective holding of power by many individuals, including commoners (Blanton et al. 1996).

Until recent decades, it was common for archaeologists and anthropologists studying ancient Mesoamerica to assume that the prevailing model was the autocratic centralization of power, which ignored the fact that there was evidence in at least some archaeological sites for wide dispersal of power, and perhaps even in some cities something like republican or democratic rule.

I thought about this a few years ago when I was touring the ruins of Tikal, which show a palace facing onto a plaza with pyramids on the side.  It's easy to see the evidence of an hierarchical society ruled by kings, priests, and nobles exploiting the commoners.  Tikal reached the peak of its power in 200-900 AD as the capital of a conquest state.  But believing that human beings have an evolved disposition for freedom and resistance to dominance, I had to wonder whether there was any sign of free societies in Mesoamerica.

                                 Tikal the Capital of a State that Conquered Tributary Regions

I was intrigued, therefore, to learn about Tlaxcallan, which has been identified by some archaeologists as an ancient republic in Mesoamerica.  When Hernan Cortes first landed on the coast of Mexico in 1519, he heard about the powerful Aztec Empire under the rule of Moctezuma.  Marching his soldiers toward Tenochtitlan, the capital city of the Empire, located where Mexico City is today, Cortes met resistance from some of the indigenous peoples, particularly the Tlaxcaltecas.  He learned that Tlaxcallan was a small polity not far from Tenochtitlan that had successfully resisted the attempts of the Aztecs to conquer them.  He finally persuaded the Tlaxcaltecas to become his allies in his campaign to defeat the Aztecs.  By 1521, the Aztecs were defeated, and Cortes claimed their Empire for Spain.  In his letters to Charles V, the King of Spain and the Holy Roman Emperor, Cortes described the people of Mexico, and he identified the Tlaxcaltecas as a freedom-loving people:

"They had tried with all their forces both by day and by night to avoid being subject to anyone, for this province never had been, nor had they ever had an over-all ruler.  For they had lived in freedom and independence from time immemorial and had always defended themselves against the great power of Mutezuma and against his ancestors, who had subjugated all those lands but had never been able to reduce them to servitude, although they were surrounded on all sides" (Cortes 2001, 66).

He described their government as

"almost like that of the states of Venice or Genoa or Pisa, for they have no overlord.  There are many chiefs, all of whom reside in this city, and the country towns contain peasants who are vassals of these lords and each of whom holds his land independently; some have more than others, and for their wars they join together, and together they plan and direct them" (Cortes 2001, 68).

He also described their market-based economy with a marketplace where over 30,000 people come to buy and sell.

In recent years, archaeologists studying the archaeological sites of Tlaxcallan have seen evidence confirming Cortes's comparison of Tlaxcallan with the Renaissance Italian republics (Fargher et al. 2010a; Fargher et al. 2010b; Wade 2017). In contrast to the political structure of the Aztec Empire--with kingship vested in members of the nobility--the archaeologists have found signs in Tlaxcallan of government by a council with members recruited from the ranks of commoners.  The socioeconomic egalitarianism of the city is manifested in the urban layout of scattered public plazas next to modest houses rather than royal palaces.  The distribution of standardized apartment buildings for common citizens and the fact that the art does not depict individual leaders or dynasties suggest that power was widely shared, and so inequality was low.

I have written about my recent visit to the National Museum of Anthropology in Mexico City (here).  One weakness in that museum's presentation of Mesoamerican history is that it displays the power of Moctezuma's autocratic rule over the Aztec Empire while remaining silent about Tlaxcallan's republican freedom and its denial of imperial rule, which expresses the evolved natural desire to be free from oppressive dominance.

Human nature was originally shaped in the hunter-gatherer state of nature that was a condition of equal liberty.  Consequently, we can expect that social orders that approach that original condition of equality and liberty will arise throughout human history.


REFERENCES

Blanton, Richard E., et al. 1996. "A Dual-Processual Theory for the Evolution of Mesoamerican Civilization." Current Anthropology 37: 1-14.

Borgerhoff Mulder, Monique, et al. 2009. "Intergenerational Wealth Transmission and the Dynamics of Inequality in Small-Scale Societies." Science 326: 682-688.

Caldaro, Niccolo. 2011. "On the Use of Contemporary 'Hunters and Gatherers' as Models for Prehistoric Patterns of Wealth Distribution." Current Anthropology 52: 265.

Cortes, Hernan. 2001. Letters from Mexico. Trans. Anthony Pagden. New Haven, CN: Yale University Press.

Fargher, Lane F., Richard Blanton, and Verenice Heredia Espinoza. 2010a. "Egalitarian Ideology and Political Power in Prehispanic Central Mexico: The Case of Tlaxcallan." Latin American Antiquity 21: 227-251.

Fargher, Lane F., et al. 2010b. "Tlaxcallan: The Archaeology of an Ancient Republic in the New World." Antiquity 84: 1-15.

Hill, Kim, and A. Magdalena Hurtado. 1996. Ache Life History: The Ecology and Demography of a Foraging People. Hawthorne, NY: Aldine de Gruyter.

Kelly, Robert L. 2010. "A Good Start." Current Anthropology 51: 109-110.

Kohler, Timothy A., et al. 2017. "Greater Post-Neolithic Wealth Disparities in Eurasia than in North America and Mesoamerica." Nature 551: 619-622.

Kohler, Timothy A., and Michael E. Smith. 2018. Ten Thousand Years of Inequality: The Archaeology of Wealth Differences.  Tucson, AZ: University of Arizona Press.

Marlow, Frank W. 2010. The Hadza Hunter-Gatherers of Tanzania. Berkeley: University of California Press.

Smith, Eric Alden, et al. 2010a. "Reply." Current Anthropology 51: 119-126.

Smith, Eric Alden, et al. 2010b. "Production Systems, Inheritance, and Inequality in Premodern Societies." Current Anthropology 51: 85-94.

Smith, Eric Alden, et al. 2010c. "Wealth Transmission and Inequality among Hunter-Gatherers." Current Anthropology 51: 19-34.

Smith, Eric Alden, et al. 2011. "Wealth Inequality in Foraging, Horticultural, Pastoral, and Agricultural Populations: A Reply to Caldaro." Current Anthropology 52; 579-580.

Wade, Lizzie. 2017. "Unearthing Democracy's Roots." Science 355: 1114-1118.

Property and Wealth Inequality Among Hermit Crabs

This video shows hermit crabs exchanging shells.  A hermit crab who is too cramped in his shell will move into a larger shell, and then the shell that he has discarded will be taken over by another hermit crab.  Sometimes hermit crabs will line up, and the shells will be passed down the line.  In Darwinian Natural Right, I wrote about this in the section on "The Normative Structure of Animal Movement," because hermit crabs are like human beings and other animals in that their behavior conforms to the same normative structure: they have natural desires, they have natural capacities for gathering information relevant to their desires, and they are naturally inclined to do whatever seems to satisfy their desires according to their evaluation of the information.  Aristotle lays out this normative structure in his biological works--particularly, in On the Movement of Animals and the Nicomachean Ethics.

Hermit crabs are small crustaceans who occupy the empty shells of dead snails.  They are most easily seen in tidal pools along ocean coastlines.  Their shells protect them from predators, reduce their physiological stress from desiccation, and promote their reproductive success.  Some shells are better than others in satisfying these desires, and as the animals grow they need to move to larger shells.  Finding the right shell means the difference between life and death, or at least the difference between being cramped or cozy in one's portable domicile.

A hermit crab will carefully inspect a new shell to assess its weight, size, and structure.  Even after moving into a new shell, the animal will continue to test the shell for suitability, and it will sometimes decide to move back into the old shell (Elwood and Neil 1992).  The process of evaluation becomes even more intricate when hermit crabs fight over shells.  Then they must assess not only the relative value of their shells but also the size, strength, and resoluteness of their opponents.  This competition often displays a hierarchy in which the most dominant crabs gets first choice of a shell.  If the dominant crab moves into a new shell, the old shell is occupied by a less dominant crab, which creats another vacancy for a third crab, and so on down the hierarchy.  Sociologists who study the social structure of "vacancy chains," in which resources are passed from one individual to another down a social hierarchy, have discovered remarkable similarities between hermit crabs occupying vacant shells left behind by more dominant crabs and human beings occupying jobs and houses left vacant by those of higher status (Chase 1991; Chase and DeWitt 1988).

We can see here the evolutionary origins of property.  Like all organisms, hermit crabs have evolved to control, preserve, and invest in their own bodies; and thus, as John Locke saw,. the sense of property begins in the self-ownership of one's own body.  This self-ownership then extends to the ownership of external resources when animals privatize those resources and thereby convert them into property.  For hermit crabs to preserve and protect their bodies, they must privatize a shell by carrying it on their body (Strassmann and Queller 2014).  (I have written about the evolutionary neurobiology of Lockean self-ownership here, here, and here.)

We can also see here the first steps in the evolutionary history of possessive behavior. Hermit crab conflicts over shells show first the priority of power, in which the dispute is settled by relative fighting power.  The second step is for the priority of power to become the priority of dominance, so that individuals can yield to those with higher status without the need for fighting.  The third step would be for hermit crabs to recognize prior possession, so that individuals learn that those with prior possession of a resource generally prevail over raiders (Tibble and Carvalho 2018).

Remarkably, Ivan Chase and his colleagues have recently reported a study that shows a distribution of these shells in one hermit crab population that follows a pattern similar to the wealth inequality in human societies (Chase et al. 2020; Preston 2019).  They collected 297 crabs from a tidal pool on a beach on Long Island, New York.  They removed the crabs from their shells.  They then weighed each crab, and they weighed and measured each shell.  Looking at how shells of different weights were distributed across the group, they saw a distribution curve that peaked around the medium-sized shells and then dropped as the shells got larger, then tapering off through the largest shells into a long tail.

They found that this inequality of wealth was similar to what one sees in small primitive human groups, such as hunting-gathering bands, but not as great as in large modern societies today.  The top 1 percent of hermit crabs owned about 3 percent of the total shell weight.  The top 1 percent of people in large human societies typically own a larger proportion of the wealth.  Among the hermit crabs, Chase and his colleagues observed, "there are no Warren Buffets or Jeff Bezoses."  But still the distributional pattern of inequality is similar.

One might think that the distribution of shell sizes among hermit crabs is determined simply by biological factors such as the survival and growth of either the crabs or the snails whose shells the crabs occupy.  Shell distribution might reflect the size distribution of surviving crabs over their life history or the size distribution of shells from dying snails.  But Chase and his co-authors did not find this to be the case.

It is common today to measure economic inequality in human societies with a number called the Gini coefficient, which ranges from 0 (if everyone in a society had exactly the same level of wealth) to 1 (if one person held all of the wealth, and everyone else had no wealth at all).  Chase and his colleagues estimate that the Gini coefficient for the hermit crabs is 0.32, which is only a little higher than some estimates of human hunter-gatherer societies as showing a Gini around 0.25, but much lower than the 0.41 Gini for the United States, and close to the 0.27 Gini for the Nordic social democracies such as Norway and Sweden (Borgerhoff Mulder et al. 2009).  Amazingly, the Gini for the hermit crabs coincides with what Thomas Piketty has proposed as the "ideal" Gini number for low inequality--0.33 (Piketty 2014, 247-60).

If even hermit crabs show such wealth inequality, does this indicate that inequality is "natural"?  And if so, what exactly are the natural causes of inequality?  Even in Locke's account of the state of nature as a state of natural equality, he recognized that all human beings could never be absolutely equal in all respects, because differences in age, talents, birth, merit, and social relationships would always elevate some above others.  Some researchers today explain inequality as arising from individual differences due to personality, training, education, or the random events of luck.  Others emphasize the importance of the intergenerational transfers of wealth, so that wealth accumulates by inheritance within some families.

Among hermit crabs, there is no intergenerational transfer of the wealth in shells.  Some of the inequality among hermit crabs might be explained by individual differences in size, fighting prowess, and dominance.  But Chase and his colleagues emphasize the transfer of shells through vacancy chains, and they imply that some similar vacancy chain process might explain human inequality.  Surely, however, there must be more than that at work in human wealth inequality.

I will have more to say about the evolution of wealth inequality in human societies in my next post.

I have written about the debates over inequality and the possible argument for "good inequality" here, here, here, here, here, here, and here.


REFERENCES

Borgerhoff Mulder, Monique, et al. 2009. "Intergenerational Wealth Transmission and the Dynamics of Inequality in Small-Scale Societies." Science 326: 682-88.

Chase, Ivan. 1991. "Vacancy Chains." Annual Review of Sociology 17: 133-154.

Chase, Ivan, and Theodore H. DeWitt. 1988. "Vacancy Chains: A Process of Mobility to New Resources in Humans and Other Animals." Social Science Information 27: 83-98.

Chase, Ivan, Raphael Douady, and Dianna K. Padilla. 2020. "A Comparison of Wealth Inequality in Humans and Non-Humans." Physica A 538: 122962.

Ellwood, R. W., and S. J. Neil. 1992. Assessments and Decisions: A Study of Information Gathering by Hermit Crabs. London: Chapman and Hall.

Piketty, Thomas. 2014. Capital in the Twenty-First Century. Cambridge: Harvard University Press.

Preston, Elizabeth. 2019. "Even Hermit Crabs Have Wealth Inequality." The New York Times, December 13.

Strassmann, Joan E., and David C. Queller. 2014. "Privatization and Property in Biology." Animal Behaviour 92: 305-311.

Tibble, Lucy, and Susana Carvalho. 2018. "Rethinking the Evolution of Property and Possession: A Review and Methodological Proposition." Evolutionary Anthropology 27: 285-296.

Six Solutions for the Darwinian Puzzle of Homosexuality

Homosexuality has been seen in hundreds of species of animals, including human animals.  For that reason, it is a mistake to argue--as Thomas Aquinas did--that human homosexuality is "contrary to nature" because no other animals engage in homosexual activity.

I have written about Aquinas and animal homosexuality here, here, here, and here.

Aquinas pointed to birds as manifesting the natural law for monogamous heterosexual mating and parental care of offspring.  Now we have evidence that at least 93 species of birds engage in homosexual behavior; and some of these birds show long-term monogamous pairing of females who jointly raise their offspring, which shows that animal homosexuality can achieve what Aquinas identified as the two natural ends of sexual mating--conjugal bonding and parental care (Bagemihl 1999, 479-655; Kotrschal et al. 2006; MacFarlane et al. 2010).

For example, this female-female Laysan albatross pair at Kaena Point Natural Area Reserve on Oahu, Hawaii, share in the incubation of eggs and feeding chicks.  Over 30% of the Laysan albatross nests from 2003 to 2012 were attended by female-female pairs (Young, Zaun, and VanderWerf 2008; Young and VanderWerf 2014).  The Laysan albatross can lay and incubate only one egg per year.  Most of the female-female pairs that fledged chicks in more than 1 year raised at least one chick from each female, so that both females had opportunities to reproduce.  Most of the chicks were fathered by males paired with other females, and these males had had sneaky copulations with one or both of the females in the same-sex pairing.  The female-female pairs raised fewer offspring on average than male-female pairs.  The annual productivity of females in female-female pairs was 80% lower than that in male-female pairs.  But still this is better than for an unpaired female to try to incubate an egg, rear a chick, and find food without any help.  Apparently, the female same-sex coupling here and with other birds is a response to a shortage of males.  In this colony of albatrosses on Oahu, 60% are female.  In these circumstances, the females who pair with another female are making the best of a bad job.  They probably wouldn't choose to pair with other females if there were enough males to go around.

This suggests that by nature the best conditions for reproducing and rearing offspring are a heterosexual pair of parents caring for their offspring, but that a homosexual pair caring for their young can be successful in circumstances where they imitate or approximate that natural standard, which would be the natural law argument for same-sex marriage and parenting.  Previously, I have argued (here) that that was the ultimate issue in the U.S. Supreme Court's decision in Obergefell v. Hodges (2015).

Even as these females engage in same-sex pairings, it's not clear that they are truly homosexual in the sense of having a durable preference for same-sex sexual behavior.  After all, their reproduction depends on copulating with males.  So they might be better identified as bisexual in being open to either opposite-sex or same-sex behavior depending on their opportunities.

Only two animal species have shown some individuals with an exclusively same-sex preference for life, even when partners of the opposite sex are available--humans and domestic sheep (Ovis aries) (Roselli et al. 2011).  Observations of domestic rams from around the world confirm that as many as 8% show a sexual preference for other rams.  And even when they are free to choose between estrous ewes and sexually active rams, some rams will show a same-sex mate preference.

Such animal homosexuality creates a puzzle or mystery for the Darwinian evolutionary psychologist.  If all animals are descendants of a long line of ancestors who were reproductively successful, and if inherited traits that impede reproductive fitness tend to be eliminated by natural selection, as Darwinian theory requires, then how can we explain the evolution of homosexuality, if homosexual animals do not engage in reproductive behavior?  If evolution by natural selection favors those traits that promote reproduction, then it might seem that homosexuality is indeed "contrary to nature."

The solution to this puzzle is to show how the evolution of homosexuality is compatible with reproductive fitness.  And although evolutionary scientists have not reached a general agreement on any one evolutionary theory of animal homosexuality, at least six such theories have been proposed (Savolainen and Lehmann 2007; Savolainen and Hodgson 2016).

1. Kin altruism selection.  Edward Wilson and some other biologists have applied William Hamilton's theory of kin selection to explain homosexuality as kin altruism (Wilson 1975, 1979; Kirkpatrick 2000).  According to Hamilton, the inclusive fitness of a trait depends on both direct fitness (reproducing through one's direct offspring) and indirect fitness (the reproductive success of one's close relatives).  In some circumstances, evolution might favor a trait that reduces one's direct fitness in order to devote resources to one's collateral kin.  This could apply to homosexuality if homosexuals promote the reproductive success of their relatives, perhaps by helping nephews and nieces.

In many societies, for example, homosexuals have become shamans or priests who could use their privileged status to help their relatives.  The word "nepotism"--from the Latin nepos for "nephew"--arose originally as the term for the Medieval and Renaissance popes and bishops of the Catholic Church who often appointed their nephews to positions of power in the Church.  As I have indicated in a previous post (here), there is a long history of the Catholic priesthood and episcopate being dominated by homosexuals.

This evolutionary theory of homosexuality as kin altruism would require evidence that homosexuals do indeed show an avuncular care for their relatives that increases their indirect reproductive fitness sufficiently to outweigh the loss of direct reproductive behavior.  Survey studies of homosexual men in Western Europe and North America have failed to find that homosexuals extend more care to their relatives than do heterosexual men (Bobrow and Bailey 2001; Rahman and Hull 2005).  By contrast, some studies in Samoa (Vasey et al. 2010; VanderLaan et al. 2013) and in Java (Indonesia) (Nila et al. 2018) have reported greater avuncular tendencies in male homosexuals as compared with male heterosexuals.  So the evidence for this theory is weak at best.

2.  Overdominance selection.  In the case of overdominance, a "gay allele" would result in homosexual behavior in an individual who has received this allele from both parents (homozygous), but an individual receiving this allele from only one parent (heterozygous) would be heterosexual and would have increased fitness over the homosexual.  This would be similar to the famous case of sickle-cell anemia in Africa: those who are homozygous for the sickle-cell allele suffer from this genetically inherited disease, but those who are heterozygous for this allele have some resistance to malaria; and therefore this allele is maintained in human populations exposed to malaria.  Although in principle this theory could explain the evolution of homosexuality, there is no clear evidence for it.

3.  Sexually antagonistic selection.  For sexually antagonistic selection, the idea is that a hypothetical gay allele would result in reduced fitness when expressed in males who become homosexual, but this would be counterbalanced by greater fertility when the allele is expressed in females.  This is one possible explanation for why some domesticated male sheep show a lifelong homosexual preference, but it's not clear whether this is true for wild sheep.  Domestic sheep have been bred by farmers to produce highly fertile females, and it could be that the genetic factors favoring fertile females also favor homosexual males.  However, there is little evidence for this.  In one experiment comparing male offspring from high and low fertility lines of ewes, there was no evidence that selection for high fertility ewes favored same-sex oriented rams (Stellflug and Berardinelli 2002).

4.  Alliance formation theory. Many animals engage in homoerotic behavior--same-sex genital contact that is experienced as pleasurable--and this often seems to strengthen the social bonding of individuals.  Bonobos, for instance, have become famous for how they use polymorphous sexual acrobatics as a social glue for binding individuals in alliances--including female-female alliances that allow the females to dominate the males.  This has led some evolutionary psychologists to speculate that homoerotic behavior could have evolved among human ancestors as a mechanism of affiliation that creates same-sex alliances that had adaptive value (Muscarella 2000).

In many human societies, adolescents and young adults go through a period of sex-segregated social and physical isolation, living on the periphery of their societies; and same-sex friendships reinforced by homoerotic behavior could have helped them move up in social status.  Moreover, these social alliances could have helped males to mate with females for reproductive benefits.  At the same time, female same-sex alliances could have helped females raise their offspring when the fathers were not around to help with parental care.  In ancient Greece and Rome, young men engaged in homoerotic friendships with older men who sponsored their social advancement.  When these young men reached full adulthood, they could marry women and form families.

The evidence for this theory is limited.  Although sometimes in some societies, homoerotic behavior can foster alliance formation, it seems to be more common for same-sex alliances to arise from friendly affection and mutual trust without homoerotic activity.

Another problem for this theory is that it's not clear that it's really about homosexuality in the sense of exclusive homoerotic preference, because it assumes that human beings have evolved to be naturally disposed to both homoerotic and heteroerotic behavior, so that same-sex friendships reinforced by homoerotic activity will lead to social alliances that facilitate success in heterosexual mating and parenting.

5. The bisexual advantage.  Another way to resolve the Darwinian puzzle of homosexuality is to deny that homosexuality really is an evolutionary puzzle.  There would be a puzzle if sexual orientation was a strictly binary trait, so that an animal must be either homosexual with no reproductive fitness or heterosexual with high reproductive fitness.  In that case, it would indeed be puzzling that homosexuality has not been eliminated by natural selection.  But if in fact sexual orientation is a continuously variable quantitative trait influenced by many genes interacting, ranging from exclusively homosexual preference through degrees of bisexuality to exclusively heterosexual preference, then there need be no Darwinian mystery about this (Sell 1997),   Continuously variable traits can have fitness costs at the phenotypic extremes, but such traits will persist because these fitness costs are low on average in evolutionary history (Lande 1976).  After all, throughout most of human history, most homosexuals have chosen to enter heterosexual marriages in which they have produced and reared children.

Defending this bisexual advantage theory, Savolanen and Hodgson (2016) explain this theory as predicting:

"that (i) some degree of bisexual attraction is more common than exclusive homosexual or heterosexual attraction, (ii) sexual attraction is fluid and changes depending upon social context (i.e., the environmental component), (iii) many genetic loci weakly influence sexual attraction, and (iv) there are no genetic loci that strongly influence sexual atrraction. . . ."

"There is some behavioral as well as genetic support for this bisexual advantage hypothesis.  First, good estimates of the frequency of bisexuality do not exist; however, the majority of human and nonhuman animals that participate in homosexual sex also participate in reproductive sex (Kirkpatrick 2000; Vasey 1995).  Second, the expression of homosexual behavior is known to be condition-dependent in humans.  For example, having spent time in the US Navy or in a British boarding school increases the likelihood of having had homosexual experience (Kirkpatrick 2000).  Finally, genomic surveys looking for genes that influence sexual attraction have failed to find any loci that strongly predict homosexuality (Sanders et al. 2014)."

6.  The epigenetic model.  Epigenetic effects are chemical modifications to DNA or to the proteins associated with DNA that don't involve changes to the DNA sequence itself.  These epigenetic markers regulate the expression of genes.  William Rice and his colleagues (2012, 2013) have developed an epigenetic model of how the transmission of epigenetic marks that influence sensitivity to androgen hormones in fetuses could explain homosexual orientation.

Typically, in fetal development, epigenetic marks make chromosomal girls (XX) less sensitive to masculinizing androgens and make chromosomal boys (XY) more sensitive.  Rice and his colleagues theorize that if the epigenetic marks directing the sexual development of the brain are not erased correctly, a mother could pass down to her son the epigenetic marks that direct the female development of the brain, so that her son develops a sexual attraction to men, or a father could pass down to his daughter the epigenetic marks for male development of the brain, so that his daughter is sexually attracted to women.

Although this seems theoretically possible, there is not yet any clear evidence to support it.


REFERENCES

Bagemihl, Bruce. 1999. Biological Exhuberance: Animal Homosexuality and Natural Diversity. New York: St. Martin's Press.

Bobrow, D., and J. M. Bailey. 2001. "Is Male Homosexuality Maintained Via Kin Selection?" Evolution and Human Behavior 22: 361-368.

Kirkpatrick, R. C. 2000. "The Evolution of Human Homosexual Behavior." Current Anthropology 41: 385-413.

Kotrschal, Kurt, J. Hemetsberger, and B. Weiss. 2006. "Making the Best of a Bad Situation: Homosexuality in Male Greylag Geese." In Volker Sommer and Paul L. Vasey, eds., Homosexual Behavior in Animals: An Evolutionary Perspective, 45-76. Cambridge, UK: Cambridge University Press.

Lande, R. 1976. "The Maintenance of Genetic Variability by Mutation in a Polygenic Character with Linked Loci." Genetical Research 26: 221-235.

MacFarlane, G. R., S. P. Blomberg, and Paul L. Vasey. 2010. "Homosexual Behavior in Birds: Frequency of Expression Is Related to Parental Care Disparity Between the Sexes." Animal Behaviour 80: 375-390.

Muscarella, Frank. 2000. "The Evolution of Homoerotic Behavior in Humans." Journal of Homosexuality 40: 51-77.

Nila, Sarah, Julien Barthes, Pierre-Andre Crochet, Bambang Suryobroto, and Michel Raymond. 2018. "Kin Selection and Male Homosexual Preference in Indonesia." Archives of Sexual Behavior 47: 2455-2465.

Rahman, Q., and M. S. Hull. 2005. "An Empirical Test of the Kin Selection Hypothesis of Male Homosexuality." Archives of Sexual Behavior 34: 962-969.

Rice, William R., U. Friberg, and S. Gavrilets. 2012. "Homosexuality as a Consequence of Epigenetically Canalized Sexual Development." The Quarterly Review of Biology 87: 343-368.

Rice, William R., U. Friberg, and S. Gavrilets. 2013. "Homosexuality Via Canalized Sexual Development: A Testing Protocol for a New Epigenetic Model." BioEssays 35: 764-770.

Roselli, C. E., R. C. Reddy, and K. R. Kaufman. 2011. "The Development of Male-Oriented Behavior in Rams." Frontiers in Neuroendocrinology 32: 164-169.

Sanders, A. R., E. R. Martin, G. W. Beechman, S. Guo, K Dawood, G. Rieger, et al. 2014. "Genome-Wide Scan Demonstrates Significant Linkage for Male Sexual Orientation." Psychological Medicine 45: 1379-1388.

Savolainen, Vincent, and Jason A. Hodgson. 2016. "Evolution of Homosexuality." In T. K. Shackelford and V. A. Weekes-Shakelford, eds., Encyclopedia of Evolutionary Psychological Science.  Springer International Publishing.

Savolainen, Vincent, and L. Lehmann. 2007. "Evolutionary Biology: Genetics and Bisexuality." Nature 445: 158-159.

Sell, R. L. 1997. "Defining and Measuring Sexual Orientation: A Review." Archives of Sexual Behavior 26: 643-658.

Stellflug, J. N., and J. G. Berardinelli. 2002. "Ram Mating Behavior After Long-Term Selection for Reproductive Rate in Rambouillet Ewes." Journal of Animal Science 80: 2588-2593.

VanderLaan, D. P., and Paul L. Vasey. 2013. "Birth Order and Avuncular Tendencies in Samoan Men and fa'afafine."  Personal Relationships 19: 326-339.

Vasey, Paul L., and D. P. VanderLaan. 2010. "Monetary Exchanges with Nieces and Nephews: A Comparison of Samoan Men, Women, and fa'afafine." Evolution and Human Behavior 31: 373-380.

Wilson, Edward O. 1975. Sociobiology: The New Synthesis. Cambridge: Harvard University Press.

Wilson, Edward O. 1979. On Human Nature.  Cambridge: Harvard University Press.

Young, Lindsay C., and Eric A. VanderWerf. 2014. "Adaptive Value of Same-Sex Pairing in Laysan Albatross." Proceedings of the Royal Society B 281: 20132473.

Young, Lindsay C., Brenda J. Zaun, and Eric A. VanderWerf. 2008. "Successful Same-Sex Pairing in Laysan Albatross." Biology Letters 4: 323-325.

Pseudocopulation Among the Echinoderms--The Evolutionary Origins of Homosexuality

Let's admit it.  We've all thought about pseudocopulation.  So let's talk about it.

One kind of pseudocopulation is male insects copulating with orchids that look and smell like female insects.  You can see some videos of this here and here.  But I warn you that they contain adult content.

Anne Gaskett at the University of Auckland and her colleagues have shown how orchids mimic female insects in ways that attract bees, wasps, and flies to copulate with them and then serve as polinators.  Most flowers attract and reward insect pollinators with nectar, but orchids attract their pollinators with deceptive signals that imitate those produced by female insects.  Orchids can even stimulate male insects to ejaculation (Gaskett et al. 2008).  This serves the reproductive fitness of the orchids, but it's costly for the insects who waste copious sperm.  Some male insects prefer orchids to real females!  Since this copulation does not actually result in sexual union, it can be called pseudocopulation.

Another kind of pseudocopulation is shown by some starfish and other echinoderms.  Starfish or sea stars are star-shaped marine invertebrates that typically have five arms radiating out from a central disc.  They reproduce both sexually and asexually.  Most species of starfish reproduce sexually, and they have separate male and female individuals that produce gametes.  Through spawning, eggs and sperm are sprayed into the water, and fertilization occurs outside their bodies.  For most species, the offspring develop on their own with no parental care.  But for some species, there is brooding, in which the offspring are attached to the mouth of the female, so that she can keep the eggs clean and healthy and protect them from predation--an amazing display of parental care in an animal that has no brain!  (See Hamel and Mercier 1995.)

Three species of starfish--Archaster typicus, Neosmilaster georgianus, and Archaster angulatus--are known to show pair mating in pseudocopulation, in which one starfish lays on top of another (Keesing et al. 2011).  During spawning events, large numbers of mating individuals mate synchronously.  This is not true copulation, because the male does not inseminate the female.  But it seems that pseudocopulatory mating ensures a high level of fertilization of eggs by sperm floating in the water.

The female is always on the bottom, and usually one male is on top of her.  But sometimes multiple males will be stacked on top of her.  So now we know the answer to a great philosophical question: Who's on top?  The male's on top, the female's on the bottom!  And sometimes the female will be crushed by a pile of males on top of her!

For more on this, you can go to the echinoblog here.

Although copulating pairs of starfish are mostly male on female, sometimes it's male on male.  In one study, a sample of 10 mating pairs were taken from the west coast of Australia, and 8 were male on female pairs, while 2 were male on male (Keesing et al. 2011).  Pairs remain in mating position for up to 24 hours.  So the pseudocopulation of these starfish show both heterosexual and homosexual behavior.

Sea urchins are another kind of echinoderm that form mating pairs.  Sea urchins have spherical bodies covered by moveable spines that make them look like little hedgehogs.  For most of the year, they live separate from one another.  But in the spawning season, they move together into pairs with their spines interlocked.  Forming mating pairs seems to increase the external fertilization of eggs by sperm.

Apparently, sea urchins cannot recognize sex differences, and so their pair mating occurs randomly.  Homosexual and heterosexual pairs occur at frequencies that would be expected by chance.  So in a population with a 1:1 sex ratio, random pairing produces male/female pairs 50% of the time.  Since the density of deep-sea echinoderms is normally low, this pairing increases the chances of successful fertilization.  In one study, 26 pairs were collected, and of these, 12 were female/male, 7 were female/female, and 7 were male/male (McCarthy and Young 2002; Young et al. 1992).  These echinoderms thus show indiscriminate sexual behavior directed towards all sexes, with both homosexual and heterosexual behavior.

Echinoderms arose as an early branching lineage of animals.  While the first vertebrates appeared about 500 million years ago, echinoderms first appeared in the Lower Cambrian geological time period--from 542 to 511 million years ago (Smith and Zamora 2013).  It is likely, therefore, that echinoderms have the traits of the ancestral organisms in which sexual behaviors evolved.  This suggests that indiscriminate sexual behavior with both same-sex and different-sex mating was the original ancestral condition for animals.

Homosexual behavior has been observed in over 1,500 animal species widely distributed across most evolutionary groups of animals (Bagemihl 1999; Poiani 2010; Sommer and Vasey 2006).  For many evolutionary biologists, this creates a "Darwinian puzzle": it seems hard to explain how homosexuality has repeatedly evolved and persisted in many different lines of evolutionary descent despite the fact that it reduces reproductive fitness.  To explain this, some evolutionary theorists have tried to show how homosexuality might have some adaptive benefits that outweigh its maladaptive costs: for example, homosexuality might have some indirect fitness benefits for the relatives of homosexual individuals.  Other theorists argue that homosexuality must be seen as a maladaptive trait that arises by accident:  for example, homosexual mating could be simply a matter of mistaken identity.

But notice the usually unstated assumption that homosexual behavior is an anomaly that requires some special explanation because it departs from an ancestral population that was exclusively heterosexual.  Recently, Julia Monk and her colleagues have challenged this undefended assumption that heterosexuality is the baseline condition for animals as showing a cultural bias for seeing homosexuality as abnormal or unnatural.  Monk and her collaborators identify themselves as "LGBTQ+ scientists," who are free from the "heteronormative bias" of most scientists.  They propose considering an alternative hypothesis that starts with a different baseline: what if ancestral sexual behavior was originally indiscriminate in mixing homosexual and heterosexual behavior, because animals were mating randomly with either same-sex or different-sex individuals?  This hypothesis of indiscriminate sexual behavior from the origin of animal sexuality would seem to be the most parsimonious explanation for what we see today--the widespread mixture of homosexuality and heterosexuality across most animal groups (Monk et al. 2019).  And they point to echinoderms as manifesting this ancestral condition of bisexuality.

Monk and her colleagues have summarized their reasoning in a blog post for Scientific American here.  They have also received a lot of press coverage including an article in The New York Times.

Despite their claim that with their alternative hypothesis homosexuality is not a "Darwinian puzzle," Monk and her collaborators must still solve the puzzle.  Even if we assume that homosexuality arose at the beginning as part of the original condition of animal sexual behavior, we must still find a Darwinian explanation for the persistence of homosexuality despite its costs for reproductive fitness.  To do that, they will have to show either that the fitness costs of homosexuality are not as high as they might seem, or that homosexuality has some indirect fitness benefits that outweigh the costs.  that will be the subject of my next post.

Some of my previous posts on animal homosexuality can be found here and here.


REFERENCES

Bagemihl, Bruce. 1999. Biological Exuberance: Animal Homosexuality and Natural Diversity. New York: St. Martin's Press.

Elbein, Asher. 2019. "Seeking a New Lens to Study Same-Sex Behavior in Animals." New York Times, November 26.

Gaskett, Anne C., et al. 2008. "Orchid Sexual Deceit Provokes Ejaculation." The American Naturalist 171: E206-E212.

Hamel, Jean-Francois, and Annie Mercier. 1995. "Prespawning Behavior, Spawning, and Development of the Brooding Starfish Leptasterias polaris." Biological Bulletin 188: 32-45.

Keesing, John K., et al. 2011. "Synchronous Aggregated Pseudo-copulation of the Sea Star Archaster angulatus Muller & Troschel, 1842 (Echinodermata: Asteroidea) and Its Reproductive Cycle in South-western Australia." Marine Biology 158: 1163-1173.

Karmath, Ambika, et al. 2019. "Why Is Same-Sex Sexual Behavior So Common in Animals?" Scientific American, blog, November 20.

McCarthy, Daniel A., and Craig M. Young. 2002. "Gametogenesis and Reproductive Behavior in the echinoid Lytechinus variegatus."  Marine Ecology Progress Series 233: 157-168.

Monk, Julia D., et al. 2019. "An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals." Nature Ecology and Evolution 3: 1622-1631.

Smith, Andrew B., and Samuel Zamora. 2013. "Cambrian Spiral-Plated Echinoderms from Gondwana Reveal the Earliest Pentaradial Body Plan." Proceedings of the Royal Society B 280: 20131197.

Young, C. M., et al. 1992. "Seasonal Breeding Aggregations in Low-density Populations of the Bathyal Echinoid Stylocidaris lineata." Marine Biology 113: 603-612.