So, for example, Thomas explains the natural law of marriage as serving two natural ends--parental care and spousal love--that can be explained by comparing human beings with other animals that show parental care by both parents. For many animals, he observes, the mother alone is sufficient to care for her offspring. But for some animals, such as some species of birds, the mother needs the help of the father or others in caring for offspring, and this is especially true where the offspring are dependent on adult care for a long time during which they need not only nourishment but also social learning if they are to flourish as adults. This is true for human beings, Aquinas argues, who are naturally inclined to be cooperative breeders, which is the natural foundation for human marriage and familial bonding.
If this is true, then we might expect that modern biological research can show the evolved neurophysiological basis for such natural inclinations to parental care. Indeed, there is now extensive evidence that human parenting behavior is facilitated by complex neuroendocrinological mechanisms. And yet, in some respects, the evidence for mother-child bonding is clearer than for father-child bonding. In general, women show a stronger propensity for parental care than men. This is true for most mammalian species. But in contrast to our closest evolutionary relatives--the great apes--direct male care for children is prevalent in many human societies. From this perspective, paternal care is a defining trait of the human species and thus a crucial factor in human evolution.
The hormonal basis for this is becoming ever clearer. In Chapter 5 of Darwinian Natural Right, I surveyed some of the research indicating that oxytocin and vasopressin support parental care and pair-bonding in mammals. Recently, there has been growing evidence for the importance of testosterone in mediating paternal care.
Lee Gettler and his colleagues have just published a new study showing how fluctuations in testosterone modulates male mating and parenting. This article has received a lot of press coverage, including an article in the New York Times. In a commentary on the article, Peter Gray summarizes this study:
"They find that, in a community-based sample from the Philippines, men with higher testosterone level are more likely to marry than men with lower testosterone; that men who marry and become fathers experience decliens in testosterone; and that men who provide more parental care have lower testosterone levels than fathers who provide less care. This is not the first study that has investigated the social dimensions to male testosterone levels. However, it represents perhaps the most rigorous study of its kind conducted on humans, and clearly demonstrates through a longitudinal design that fatherhood causes testosterone decreases in men."High levels of testosterone make men more attractive to potential mates and make them more aggressive in competing with other men for mating opportunities. Lower levels of testosterone are associated with marriage and child care. Thus, fluctuations in levels of testosterone seem to mediate the tradeoffs between mating and parenting.
For Gettler, this supports his larger theory of human evolution based on a "metabolic model of direct male care." Evolutionary theorists have noticed the shortened interbirth intervals that characterize modern humans as compared with the great apes. Gettler argues that this was made possible by hominid males helping mothers by helping in the carrying of the young, and thus lessening the costs of child care by mothers. Sarah Blaffer Hrdy and others have argued that a distinctive feature of human evolution was "alloparenting"--people helping mothers with child-care and thus reducing the burden of maternal caregiving. Gettler emphasizes the importance of direct paternal care for offspring as a crucial part of this alloparenting. The connection of testosterone to male mating and parenting is one proximate mechanism that has emerged from the human evolution of sexual mating and parental care.
This illustrates how modern evolutionary biology can explain the biological roots of human mating and parenting as natural inclinations that constitute Thomistic natural law.
Lee Gettler, "Direct Male Care and Hominin Evolution: Why Male-Child Interaction is More than a Nice Social Idea," American Anthropologist, 112 (March 2010): 7-21.
Lee Gettler, Thomas W. McDade, Alan B. Feranil, and Christopher W. Kuzawa, "Longitudinal Evidnence that Fatherhood Decreases Testosterone in Human Males," Proceedings of the National Academy of Sciences, Early Edition, 2011.
Peter B. Gray, "The Descent of a Man's Testosterone," Proceedings of the National Academy of Sciences, Early Edition, 2011.
Some related posts can be found here, here, here, and here.