• August 22, 2014

Darwin's Daddies

Fatherhood and Evolution 1

Michael Glenwood for The Chronicle Review

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Michael Glenwood for The Chronicle Review

A tremendous amount of information is available on human parenting. There are books and more books, journal articles and more journal articles. Yet the vast amount of the scholarship on human parenting focuses on maternal behavior. Browse the nearest library, bookstore, or Web site, and you will find that to be the case.

That does not mean that a valuable body of research on human fatherhood does not exist. It certainly does. However, much of the published work, especially that intended for popular audiences, seems to assume that men will read it only if it is interspersed with jokes, sports, and sex. Maybe book publishers feel that more humor than research content is needed to appeal to men's minds. Such books can be enjoyable; we find humor and truth in works like Robert Wilder's Daddy Needs a Drink: An Irreverent Look at Parenting From a Dad Who Truly Loves His Kids—Even When They're Driving Him Nuts (Delacorte Press, 2006).

A different type of work, both scholarly and popular, takes another approach. These books and articles on human fatherhood discuss excellent empirical research but commonly conclude with advice or advocacy—prescriptions for individual parenting styles or for political issues related to fatherhood, like family leave. Well-received books on fatherhood like The Expectant Father: Facts, Tips, and Advice for Dads-to-Be (Abbeville Press, 1995), by Armin A. Brott and Jennifer Ash, represent that approach.

Unlike such works, we do not offer advice. Rather, we want to give you a brief synthesis of research on human fatherhood—and the many questions still to be answered.

Our interest, in particular, is in a body of scholarly research on human fatherhood undertaken from evolutionary and cross-cultural approaches. This research tells us that in only 5 percent of species of mammals do males provide parental care. In addition, in all human societies, men spend less time in direct child care than women do. In hunter-gatherer societies, men tend to spend more time with young children than they do in most other societies, especially ones in which men make a living herding livestock. A variety of studies have also found that men tend to invest more in children if they are the biological parents rather than the stepparents. And a handful of recently published works observe that fathers tend to have lower testosterone levels than do men without children in the same society. Such findings tend to be scattered across journal articles and edited volumes. Read Barry S. Hewlett's edited collection of essays, Father-Child Relations: Cultural and Biosocial Contexts (Aldine De Gruyter, 1992) or various books edited by the fatherhood-research guru Michael E. Lamb, like his much-republished The Role of the Father in Child Development (Wiley, 1976), and you will find the data.

What issues do the results raise? One source of information is comparative research, or contrasts across species. Because we are primates, we find most helpful those references comparing us with nonhuman primates, especially our closest living relatives, the apes. Among the apes, we are most closely related to chimpanzees and bonobos, followed by gorillas, then orangutans, and then gibbons and siamangs. Males in those species differ from us in their paternal proclivities. Although approximately 90 percent of bird species are socially monogamous (a single male and female bonded together, with or without sexual fidelity) and have males that invest in offspring, paternal care is largely absent among our primate cousins. Take the abundant and pesky rhesus macaque or vervet monkey. Very few males of those species provide any meaningful amount of paternal care. That is the standard protocol for Old World monkeys.

What about the lesser apes, like gibbons and siamangs? They tend to form social systems including one adult male, one adult female, and their offspring; adults defend their territories from intruders of their own species that might seek to mate with their partners. Sometimes they do engage in extra-pair mating, although they are generally monogamous. The males, however, are not that involved in paternal care. And the great apes? Do they provide parental care? The quick answer is no.

Similarly, the fossil record of our ancestors from around six million years ago suggests that at the start, they had no glimmer of paternal care in their eyes. What changed?

The amount of fossil material recovered about our hominin forebears and cousins from, approximately, the last six million years (since our last common ancestor with chimps and bonobos) is small but growing. Combined with information on dates, developmental patterns, and material culture, like stone-tool kits, the collective body of data provides some inferences regarding the evolutionary history of human paternal care. First, parental behavior today has been derived recently in evolution. Second, consistent with mammalian patterns of mating systems and paternal care, human pair-bonds may have originated in males' guarding their mates. We see that process getting under way around two million years ago but becoming more pronounced around a half a million years ago. As our human ancestors adopted fully bipedal ways in more-open environments and increased the proportion of their diet obtained from meat sources, group sizes may have increased, resulting in a unique form of multi-male, multi-female social group (rather than, as in the pattern of gibbons, territorial bonds in family groups). In those groups, we see an important role for males—but also for female relatives, like older sisters and adult females, in providing direct care of children.

The formation of long-term bonds would also permit other social changes. For example, assuming that couples slept next to each other at night, fathers would find themselves in proximity to their offspring, possibly enhancing the development of an attachment to them. Reductions in mortality may also have allowed more fathers to survive to watch their children grow and thus to play more important roles in socialization.

However the precise evolutionary details of human paternal care unfolded, it is clear from a comparative perspective that paternal investment is a distinguishing characteristic of our species. It is equally obvious that human paternal care exhibits differences both cross-culturally and within cultures.

The cross-cultural and historical record on human paternal care and related facets of male behavior is important for several reasons. One, what happens in the United States or Jamaica or South Africa or China is not representative of all humanity. Rather, aspects of human fatherhood show some similar patterns across a wide range of societies, but they show clear variation, too. One of the issues we need to better understand is the bases of that variation, like why the mode of subsistence in a society can structure the amount and kinds of care that fathers provide their children.

Within human variation, we can gain special insights from hunter-gatherer societies, in socioecological contexts similar to those in which paternal behaviors arose. Until the invention of agriculture, a mere 10,000 years ago, our ancestors were hunter-gatherers who lived in small-scale, face-to-face societies; accordingly, we can profit by studying the ethnographic descriptions of such contemporary societies. That does not mean that recently studied hunter-gatherers are "living fossils." They are fully modern human beings, often interacting with their non-hunter-gatherer neighbors, with tool kits far more advanced than those of their and our more-distant hunter-gatherer ancestors. All that might affect the hunting ability of such men and our understanding of male roles, including fatherhood. Still, an evolutionary sense of human fatherhood modeled on the socioecology of hunter-gatherers gives us a far better starting point than, say, a large, highly stratified society in which an emperor like Genghis Khan might father hundreds of children across his life span without knowing their names.

Consider the hunter-gatherer Hazda, in northern Tanzania. Fathers in camps spend around 5 percent of their time holding their children 9 months or younger. That is fairly consistent with data from other societies in Africa and Australia. The main exception is the Aka, hunter-gatherers in the Congo Basin rainforest in central Africa, among whom fathers hold their children for 22 percent of their time in camp. Studies have shown that fathers in hunter-gatherer societies provide more direct infant care than do fathers in horticultural, agricultural, or pastoral societies. Further, the more polygamous the society, the less direct paternal care is provided.

A number of factors account for those patterns. Among the Aka, for example, men and women cooperate in using nets to hunt. Those activities mean that spouses are often in close contact, so that a nursing infant can be readily handed to a father. Because the subsistence nature of Aka society requires cooperation, fathers can also afford to be indulgent with their young children (the children do not need to be trained to rise in a highly competitive social hierarchy). The greater the polygamy within a society, the more men may be spending their efforts seeking additional mates rather than investing in their offspring, which would account for the negative link between polygamy and direct paternal care.

Research data also include large-scale, nationally representative studies of paternal care in the United States, Japan, India, Brazil, and elsewhere. Men in India, for example, are near their children an estimated three to five hours a day. At the other extreme, Japanese fathers average about 20 minutes. We don't know all the reasons for such differences. Age of the child could be relevant. Men in the United States, for example, may decrease their involvement as children get older: Given more-limited family-care networks and the shorter duration of mothers nursing their children than in some other societies, American men may spend more time with their highly dependent young children but less as offspring grow older and need less direct care. Another factor could be the gender of the child (fathers might have sex-specific reasons for spending time with children, for instance teaching boys to hunt).

Whatever the underlying issues, one pattern is robust across all samples in which direct care has been assessed: There is not a single society reported in which men's time devoted to direct care of young children surpasses the time women devote. That reminds us of an obvious fact: Fathers are not mirror images of mothers. Instead, fathers face parenting with the evolved baggage of a male mammal, including weighing the optimal use of their time for mating versus parenting.

Guided by the conceptual logic of natural selection, evolutionary theory suggests that biological fathers invest more in human paternal behavior than stepfathers do. Spending time on offspring who do not carry any of your genes seems counterintuitive. What work has been done testing that expectation?

Stepparenting, while not the norm among nonhuman animals, is not unheard of. Many studies, mostly in birds, have reported cases in which incoming males tolerate or provide care for existing offspring of female mates. Experimental studies, mostly involving bird species, have demonstrated that when a resident parent is removed, the replacement parent provides care for (or at least tolerates) the existing young. Several other primate species exhibit similar patterns.

Moreover, among humans, while it's true that there has been a tremendous growth in stepfamilies in contemporary industrialized societies, resulting from growing rates of divorce and births to unmarried mothers, stepparents are not a modern invention. Among the Aka, for example, 18 percent of children 11 to 15 live with mothers and stepfathers, and 6 percent live with fathers and stepmothers. That suggests that stepparenting may go far back in hunter-gather societies.

Still, the evolutionary expectation that men will be less involved with their stepchildren than with their genetic offspring has been reinforced on some issues. For example, it turns out that in terms of a variety of social and behavioral outcomes, stepchildren are more likely to be like children living with single mothers than they are to be like those living in households with two biological parents. In terms of direct parental care, many studies have also shown that stepchildren typically receive less investment from stepfathers than from genetic fathers with whom they live. Stepchildren are less likely to go to college than are genetic offspring of current mates, and among those who do go to college, stepchildren receive less money for their education.

There are multiple theories from the social and behavioral sciences on the impact of stepparenting on families­—and on fatherhood. But the evidence is at best equivocal. Nevertheless, an evolutionary perspective provides some insight into why the stepfather/stepchild relationship can be so difficult—and the fact that many men are willing to give it a try.

Another source of information is human genetics. Genetics has provided the traditional means of accounting for inheritance in an evolutionary framework, providing the mechanism of inheritance for traits evolving under natural selection. As anthropologists, we are particularly aware that inheritance does not equal genetics. Culture can also be inherited; so, too, can epigenetic information on molecular modifications to genes, or information of biological modifications of genes as a result of social or other cues. Research on male infertility, for example, deals with the effects of both genetic and epigenetic mutations and with cultural influences, like the age at which men marry and seek to have children.

The neuroendocrine system attunes an organism's perceptual, cognitive, emotional, and motor mechanisms to facilitate behavior. A handful of recently conducted MRI studies provides insights into the neural bases of human paternal care. Such studies help us see how babies can crawl into paternal brains. For example, research on the response of a subject's brain to a baby crying or laughing has found that both men and women exhibit more-pronounced neural responses in the right amygdala—the structure active in emotional processing, especially fear—to an infant crying than to one laughing. It is possible that the cry produces a response calling forth parental vigilance.

Moreover, the endocrine system consists of hormones released throughout the body to help organize, regulate, and respond to its world. Leave it to voles (small, furry rodents) to illustrate some of the specific ways in which hormones facilitate paternal behavior. Some studies have shown, for example, that changes in levels of vasopressin (the hormone that is classically recognized to increase blood pressure and act as an antidiuretic) can lead to enhanced interest in spending time with a single partner, with all the implications of pair-bonding for fatherhood.

There cannot be a definitive account of fatherhood from an evolutionary perspective. Many questions remain unanswered. New technologies that can annul the paternal relationship—like paternity tests—raise important questions about the nature of fatherhood. So do assisted reproduction and treatment for infertility. What does assisted reproduction mean for gay men? What does it mean that, for the first time in human evolutionary history, a woman can gestate a fetus created from another woman's egg? And then there's the little blue pill. The release of Viagra and related drugs to treat erectile dysfunction has raised the possibility of many men becoming fathers when they otherwise would not have.

Some of the answers will become clearer in the future. Paternal behavior is a rapidly growing research area. Whether you're a father or mother (and thus have a vital stake in male parental care), or you simply have a father yourself, you can be sure that you will be seeing fathers in a new way in the future.

Peter B. Gray is an associate professor of anthropology at the University of Nevada at Las Vegas. Kermyt G. Anderson is an associate professor of anthropology at the University of Oklahoma. This essay is adapted from their book, Fatherhood: Evolution and Human Paternal Behavior, published this month by Harvard University Press.

Comments

1. bdr8y - May 14, 2010 at 08:57 am

Most of the anthropological references cite sociological practices not evolutionary biology. What is more, it's hard to ignore the human ability to cognate when attempting to correlate our behavior with primates.

Deadbeat dads breathe easy, its only science at work. It's not your fault. Mother's, keep up the thankless work, it's what you were born to do.

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