Tuesday, March 29, 2016

Is the Demographic Transition the "Central Theoretical Problem of Sociobiology"?

If human beings have evolved to have a natural desire to care for children, as I have argued, it might seem strange that in modern industrialized societies, people tend to have small families.  One might expect that the increasing economic resources in such societies would allow people to satisfy their desire for children by having large families.  The decline in the total fertility rate in industrialized societies, which began in the 19th century, is called the "demographic transition."  Over the past 50 years, some social scientists have identified a second demographic transition, in which the decline in fertility in some of the richest societies has fallen below the reproductive rate necessary to replace the present generation, which suggests that possibility that the population of some socially successful groups will decline to the point of extinction. 

This might seem to deny my claim about parenting as a natural desire.  More generally, it might seem to deny the claim of sociobiology that human social behavior has evolved to favor genetic fitness.  Among many animal species and among human beings in most societies until the past two centuries, the most dominant human beings have tended to be more reproductively successful (Ellis 1995; Betzig 1986).  But in the most developed societies today, the rich and powerful tend to have low fertility rates, and it's the lower class people who have higher fertility rates.  That social success does not correlate with reproductive success after the demographic transition has been identified as the "central theoretical problem of sociobiology" (Vining 1986).

And yet I see the demographic transition as a natural expression of the prudent flexibility of human beings in adapting their parental desires to changing ecological circumstances.  Because of the natural variability in human temperament, some human beings will choose to be childless.  But most human beings in all societies will have a strong natural desire to care for children.  In the socioeconomic circumstances of modern industrialized and technologically advanced societies, parents will want to have small families, so that they can invest resources in their own education, in their careers, and in the education of a few children, and so that those children can become socially successful adults.  Most parents will desire to have no more than two or three children, and where mortality rates are low, this will be enough to sustain current population levels. 

In some cases where the costs of children have become very high, fertility rates have fallen below replacement levels.  In most cases, I suspect, these fertility rates will eventually return to the level of replacement.  If they do not, then those population groups with such low levels of reproductive success will eventually disappear, and thus biological evolution by natural selection will reassert itself.

The total fertility rate (TFR) is a measure of an imaginary woman who passes through her reproductive life subject to all the age-specific fertility rates for the ages 15-49 that were recorded for a given population in a given year.  So this rate is the number of children a woman would have if she was subject to prevailing fertility rates at all ages from a single given year, and if she survived throughout all her childbearing years. 

According to United Nations data, the average TFR for the world was 4.95 for 1950-1955, 3.84 for 1975-1980, 2.79 for 1995-2000, and 2.36 for 2010-2015.

If there were no mortality until the end of a woman's childbearing years, the replacement level of TFR would be around 2.0.  The replacement fertility rate is close to this for most industrialized countries.  For the global average, the TFR at replacement is about 2.33.

In the U.S., the TFR was at 2.1 in 1930-1934.  It peaked at about 3.8 in the late 1950s (during the baby boom).  Today, the TFR for native born Americans is below replacement and above replacement among immigrant families.  But the fertility rates of immigrants decrease sharply in the second generation, which is correlated with greater education and income in the second generation.

The average TFR for the European Union is 1.59.  The lowest TFR for any country today is .81 for Singapore.

The lowest TFR in recorded history is .41 for the Xiangyang district of Jiamusi city in China.  Outside China, the lowest TFR ever recorded was .80 for East Germany in 1994.  The East German case is an extraordinary example of how fast people can change their fertility decisions in response to changing historical conditions (Conrad, Lechner, and Werner 1996).  Both West and East Germany had a post-war baby boom that peaked at a TFR of 2.5 in 1965.  But by the early 1970s, the TFR had fallen to around 1.4, well below replacement levels, and one of the lowest fertility rates of any society in the world.  But then the Honecker regime established pronatalist policies for East Germany, including financial incentives and social benefits for marriage and births, and the TFR for East Germany rose to 2.1, while that for West Germany remained at 1.4.  Then, after the fall of the Berlin Wall in 1989 and the reunification of Germany in 1990, the TFR for East Germans dropped well below that for West Germans, apparently because East Germans thought the future socioeconomic circumstances would not be favorable for bearing children. 

Today, the TFR for the East Germans is at the level for West Germans--1.44--which is well below replacement levels.  Some Germans have worried that if this continues into the future, the native born German population could be extinguished, and the German population would be composed totally of recent immigrant groups. 

Remarkably, there is no single explanation for the demographic transition that is generally accepted among social scientists.  There are at least three theories that have been proposed (Sanderson 2014, pp. 195-200).

One theory proposed by Marvin Harris and others is that people adjust their fertility according to the economic value of children's labor.  In agricultural economies, the reasoning goes, children are valuable workers for farm families, and consequently couples will want to have many children.  But in industrialized societies, children have little value as laborers, and they become costly to rear, so that couples choose to limit the number of their children.

The problem with this theory is that there is little evidence that in high fertility societies, wealth flows from children to parents, because even in such societies, children are consuming more than they are producing, and so they are costly to rear.

Another theory is that low fertility societies are those in which women have more autonomy, and since the burden of producing and caring for children is usually greater for women than for men, empowered women will choose to have fewer children than when men are in control.

But while there is evidence of some correlation between low fertility and female empowerment, it often seems that female empowerment is more the effect than the cause of low fertility (Sanderson 2001, 168-76; Sanderson and Dubrow 2001).  As the fertility rate drops, women have greater freedom to pursue the education and career activities that empower them.

A third theory seems to me to be most plausible in being supported by extensive evidence (Arnhart 1998, 116-118).  It's what Hillard Kaplan and his colleagues have called "the embodied capital theory of life history evolution" (Hill 1993; Kaplan 1996; Kaplan et al. 2002; Kaplan et al. 2003; Lancaster 1997).

This theory shows how a Darwinian view of parental investment can explain the cultural variation in parental behavior.  Human parental investment in children is neither genetically fixed nor culturally arbitrary.  Parental investment is not genetically fixed, because the human desire for parental care manifests itself in variable ways in response to the variable ecological conditions of the physical and social environment.  And yet parental investment is not culturally arbitrary, because its variability follows a predictable pattern as human beings strive to satisfy their natural desire for parenting in diverse circumstances.  As Aristotle would say, parental care of children is natural, but prudence dictates that the best way to satisfy that natural desire varies to conform to the conditions in which human beings happen to find themselves.

Patterns of parental investment emerge from a series of decisions involving trade-offs that all animals (including human beings) must make (either consciously or unconsciously) over the course of their life history.  These trade-offs require allocating scarce resources (such as time, energy, and risk) between competing activities. 

The first trade-off is between somatic effort (investing resources in the growth, development, and maintenance of the individual animal) and reproductive effort (investing resources in the individual animal's offspring).  The second trade-off is between two forms of reproductive effort--mating effort (investing resources in the search for mates) and parental investment (investing resources in the care of offspring).  The third trade-off is between two forms of parental investment: one stresses the quantity of offspring (investing in a large number of offspring but with each receiving few parental resources), and another stresses the quality of offspring (investing in a lesser number of offspring but with each receiving extensive parental resources).  All animals must make such choices (consciously or unconsciously), and whether one choice is better or worse than another depends on the variable ecological conditions of their lives (Emlen et al. 1995).

Like other animals, all human beings must decide how much to invest in themselves as compared with their children, because producing and caring for children is costly.  All human beings who decide to reproduce must then decide how much to invest in finding mates as compared with investing in the children produced by mating, because directing resources to finding new mates can mean decreasing the resources available for the children produced through earlier mating.  All human beings who decide to invest care in their children must then decide whether to divide their resources among many children or concentrate their resources on a few children, because children typically consume more parental resources than they produce, and therefore any increase in the quantity of children requires some reduction in the quality of the resources available for each child.  Deciding between the quantity and quality of children will be influenced by the mortality rate, because if many children die before they reach mature adulthood, parents will be less inclined to invest great resources in them.  The low mortality rate in modern industrialized societies is one reason parents can invest their resources in a few children without much fear of losing the return on their investment.

The differences in how people make these decisions manifest the complex interactions of differences in individual temperament, individual history, sexual identity, and social history.  Although most human beings will become parents at some time in their lives, a few will never become parents, either because they lacked the opportunity (through infertility or failure to find a suitable mate), or because by temperament and circumstances their parental desire was weak in comparison with other desires.  Men will tend on average to invest somewhat more in mating effort and somewhat less in parental care than do women on average, because while a man can impregnate many women, women cannot be impregnated more than ten to twenty times in their lives.  Yet despite this difference in tendency between men and women, most men desire the social stability that comes from mating with one woman or a few women and then investing great paternal care in the children produced.

The choices human beings make about parental investment will also reflect social history, because the likelihood that one choice is better than another in satisfying the natural desire for parental cadre will depend on social conditions that determine the costs and benefits of alternative reproductive strategies.  One example of such social variability is in the choice between the quantity and quality of children.  As a general rule, animals in good condition (having good health, plentiful physical resources, or high social status) tend to produce more offspring than animals in bad condition.  This seems to have been true for human beings throughout most of their history: the healthier, wealthier, and more powerful people have tended to have more children that survive to adulthood.  But now a new pattern has emerged.  People in modern, industrialized societies tend to produce fewer children on average than people in less developed societies.

One can explain this in Darwinian terms.  In the developed societies, the economic and social success of children depends ever more on their acquired technical skills and educational training, so that as adults they can compete for jobs that require special talents and knowledge. Consequently, rearing successful children in such societies requires that parents make increasingly costlier investments in the education of their children.  And as the cost of children has thus increased, the quantity of children demanded by parents has decreased, because parents now express their natural desire for parenting successful children by investing more resources in fewer children, thereby choosing quality over quantity in their children.

Moreover, for human beings to be able to invest so much in their children, they must first invest time and energy in their own education and career development and in the search for suitable mates.  Consequently, successful people in modern developed societies must often delay their marriage and having children, and thus the average ages for getting married and having the first child have risen.  When women do this, they sometimes reach the end of their reproductive years, and so produce fewer children than they would have desired.  It's likely that one reason for the total fertility rate falling below replacement levels is that couples who desire two or three children delay marriage and reproduction for so long that the women reach the end of their physiological fertility sooner than they had expected.  Then couples must rely on fertility treatments or adoption to satisfy their parental desires.

This shows the kinds of decisions that human beings must make as they adjust their patterns of parental investment to be adapted to the ecological circumstances in which they find themselves.  A natural human desire for parental care will be culturally and individually variable because of the variability in the physical and social conditions of parental care and in individual personality.  Consequently, the evolutionary behavioral ecology of parental care must move through three levels of analysis--the genetic history of the species, the cultural history of the group, and the individual history of those making their decisions about parenting.

I need to say more about "embodied capital theory," which I will do in my next post.


Arnhart, Larry. 1998.  Darwinian Natural Right: The Biological Ethics of Human Nature. Albany: State University of New York Press.

Conrad, C., Lechner, M., Werner, W. East German fertility after unification: crisis or adaptation?. Population And Development Review. 1996;22:331–358.

Emlen, S. T., P.H. Wrenge, and N. J. Demong. 1995. "Making Decisions in the Family: An Evolutionary Perspective." American Scientist 83: 148-57.

Hill, Kim. 1993. "Life History Theory and Evolutionary Anthropology." Evolutionary Anthropology 2: 78-88.

Kaplan, H. S. 1996. "A Theory of Fertility and Parental Investment in Traditional and Modern Human Societies." Yearbook of Physical Anthropology 39:91-135.
Kaplan, H.S. and Robson, A. 2002. The emergence of humans: The coevolution of intelligence and longevity with intergenerational transfers. Proceedings of the National Academy of Sciences 99: 10221-10226.

Kaplan, H.S., Hill, K.R., Lancaster, J.B., and Hurtado, A.M. 2000. A theory of human life history evolution: Diet, intelligence, and longevity. Evolutionary Anthropology, 9: 156-185.
J. B. Lancaster (1997) The Evolutionary History of Human Parental Investment in Relation to Population Growth and Social Stratification. P. A. Gowaty, Ed. Feminism and Evolutionary Biology. Chapman & Hall, New York. Pp.466-489.
Sanderson, Stephen K. 2001. "Explaining Monogamy and Polygyny in Human Societies." Social Forces 80:329-336.
Sanderson, Stephen K. 2014. Human Nature and the Evolution of Society. Boulder, CO: Westview Press.
Sanderson, Stephen K., and Joshua Dubrow. 2000. "Fertility Decline in the Modern World and in the Original Demographic Transition: Testing Three Theories with Cross-National Data." Population and Environment 21:511-537.
Vining, Daniel R. 1986. "Social versus Reproductive Success: The Central Theoretical Problem of Human Sociobiology." Behavioral and Brain Sciences 9:167-216.


Anonymous said...

Regarding energy invested in finding a mate versus child rearing, this article says men with a number of wives neglect their children



Another factor I think is at work is that in pre-modern societies, children take care of their parents when they grow old, but in modern societies that is handled by other means such as pensions and Social Security.

Larry Arnhart said...

Yes, the competition among polygamous wives for the husband's investment in their children is one of the problems with polygamy. For this reason, Aquinas thought polygamy was only "partly natural," while monogamy was "fully natural" as a form of marriage.

Anonymous said...

1. Are people who express no desire for children mentally defective?
2. You keep mentioning "modern industrialized and technologically advanced societies." Why not call them liberal societies? Would that undermine your whole raison d'etre that Darwinism supports classical liberalism? Is modernism anti-Darwinian since it lowers fertility? Thus, if we were looking for a political/economic system that is supported by Darwinism ought we look elsewhere that modern liberalism?
3. "It's likely that one reason for the total fertility rate falling below replacement levels is that couples who desire two or three children delay marriage and reproduction for so long that the women reach the end of their physiological fertility sooner than they had expected." So if we are looking for a Darwinian supported society, ought there be state intervention in the private sphere (aka anti-liberlism) in order to boost fertility?

Les said...

@Anonymous: regarding your second point, my understanding is that industrialized nations that have authoritarian governments also very often experience a great decline in fertility.