If evolutionary analysis has any salience for law, it is likely to have important insights for family law. Families, after all, are about the most biologically based of human activities: sex and reproduction. Indeed, some would say the very concept of family starts with a biological construction rooted in the ordering and regulation of biological relationships. Yet, precisely for this reason, evolutionary analysis, to be influential, must overcome resistance to the very idea of the undertaking. Some of these objections are present whenever the issue of evolutionary analysis of law is raised, but they are often asserted with more emotion in the context of the family for understandable reasons.

Take, for example, one of the most basic objections, that of the naturalistic fallacy. This doctrine posits that an ‘ought’ should not be derived from an ‘is’. Evolutionary analysis might suggest, for example, that: (1) it takes a man and a woman (or at least an egg and a sperm) to produce a child; (2) adults are more likely to look out for children who are biologically theirs; (3) men and women tend to form pair-bonds that facilitate reproduction; (4) human children take longer to raise than the offspring of other closely related species; and (5) the pair-bond is therefore likely to have arisen to facilitate human childrearing. Some authors might then conclude that children should be raised in two-parent biological families, and law should stigmatise and discourage the formation of single-parent, same-sex, step-parent or other less favoured families. While almost all scholars recognise that any simple-minded move from evolutionary observations to societal prescriptions is a misuse of evolutionary analysis,Footnote ¹ the very fact that the family orders indisputably biological activities inflames fears of misuse. After all, any number of commentators who do not pretend to endorse evolutionary analysis argue that the family of biologically related mothers, fathers and children is naturally (if not divinely) ordained, and certainly better for the well-being of children. In the context of such a debate, greater emphasis on biology, critics fear, can only serve to increase intolerance for alternative families.

We believe that such fears are well founded, and we and many others who would undertake evolutionary analysis have addressed them.Footnote ² In this article, however, we wish to take on a different issue, that is, whether at the end of the analysis, after all of the qualifications and disclaimers have been added, evolutionary analysis has anything to add, and if so, whether the insights it supplies make it a worthwhile subject of endeavour for law professors. Take, for example, the issue of family composition. Casual observation suggests, without any evolutionary analysis at all, that: (1) it takes a man and a woman (or at least an egg and a sperm) to produce a child; (2) adults are more likely to look out for children who are biologically theirs; (3) men and women tend to form pair-bonds that facilitate reproduction and child-rearing; (4) human children take longer to raise than the offspring of other closely related species; and (5) children whose parents stay together and care for them tend to do better than children whose parents do not. Whether or not these observations are true, however, is more a matter for conventional sociology than a biology lab. Studies show, for example, that American children who are adopted or whose mothers are widowed do about as well as children in conventional two parent families, but the same is not true of either hunter-gatherers (where the survival of both parents may have a dramatic impact on the children’s life expectancy) or children raised by never married Americans. Moreover, even determining that it is sometimes true that two parent biological families produce better outcomes for children raises a host of additional causal issues. Never-married mothers, for example, are different from widowed mothers in ways that may be more important than the fact they head a single parent family. Within the context of such a complex debate, what can evolutionary analysis add for law professors?

Owen Jones has provided the most considered answer. He argues that the most critical contribution evolutionary analysis makes is what he calls the ‘Law of Law’s Leverage’:

We believe that Jones’s statement is correct both in its description of human behaviour, and its conclusion that correctly identifying human behaviour likely to be resistant to change is a worthwhile endeavor. The problems arise, however, in determining: (1) when we want to develop an appropriate ‘legal intervention’; and (2) whether a ‘predisposition’ that contributed to the behaviour was ‘adaptive’. And these issues may be more central to family law than the origins of the behaviour at issue. We are thus not certain how useful Jones’s law concerning evolutionary analysis will turn out to be for law professors.

In this paper, we plan to describe our own adventures in considering the role of evolutionary analysis in family law. In the process, we have learned a fair amount, educated others, and become cautious proponents of the enterprise. Our experiences, however, have convinced us that the most important question is not whether evolutionary analysis is necessarily accurate, precise or provable, but whether it takes us down a worthwhile path of inquiry along which we would otherwise not have travelled.

Our ultimate answer is that for evolutionary analysis to be useful we must lower our expectations. We believe that evolutionary analysis is unlikely to provide a wholesale substitute for other social science techniques or for traditional legal analysis. Instead, at its best, it may lead us to ask different questions, to adopt different techniques, and to continue to question the received wisdom of more conventional research. Once we have formulated the right questions, the process of answering them might not be different at all from conventional legal, sociological or ethical analysis involving positive descriptions of behaviour followed by norm development. The question must therefore be whether these new questions are worth asking. We present our own ventures in family law as support for a tentative conclusion that they are.

In this paper, we will describe our initial family law hypothesis, the time sink involved in exploring the full range of data that might support our speculation, the risks of over-generalisation and reductionism to be avoided, the refining of the question into a more productive form, and the final result of the inquiry: commitment to an exciting new project that involves only the slightest incorporation of biological inquiry.

I. Evolutionary analysis: law professors beware

We began our inquiry into an evolutionary analysis of family law with a simple hypothesis.Footnote ⁴ We speculated that the evolutionary literature would show that the pair-bondFootnote ⁵ is a universal of the human species, but that long-term fidelity is not. We expected to conclude, therefore, that while people could be expected to readily form pair-bonds, it would require far greater societal intervention to produce long-term fidelity and that, indeed, the history of ‘civilisation’ would bear us out.Footnote ⁶

We approached our hypothesis in a manner that evolutionary analysts might have recommended: we tried to determine the evolutionary conditions that produced the pair-bond in the first place. Doing so may have been a mistake on two grounds. First, we took the universality of the pair-bond as a given, and considered ‘universality’ to mean that couples form pair-bonds in virtually all societies, and that virtually all (90% or more) individuals who have the opportunity to form a pair-bond will do so at some time in their life. The interdisciplinary sources we consulted supported the assertion and no legal audience has questioned it.Footnote ⁷ When we presented the work to an audience that included anthropologists, however, the first question was whether we maintained that such a thing as human nature existed? We think the question is a good one that it has taken us a long time to answer. Our starting assumption that the pair-bond is a universal of the species was intended as a positive assertion that the human pair-bond is species typical behaviour. Evolutionary biologists start with an assumption that closely related species exhibit different behaviour because of genetic differences that arose in the context of the factors related to the divergence from a common ancestor.Footnote ⁸ Some anthropologists, however, maintain that all human behaviour is culturally determined, and reject any assertion that human behaviour differs from that of chimpanzees because of innate characteristics. To engage the research on evolutionary biology required acceptance of the notion that species typical behaviour exists and has genetic roots. After some consideration, however, we have concluded that as law professors we have no basis on which to take a position in the controversy, and the assertion that such a thing as human nature exists may not be necessary to our family law conclusions. Nonetheless, we remain troubled by the question of how many interdisciplinary controversies are embedded in our assumptions, and how many we need to resolve to have a credible product. Law professors who start with the question of whether a given behaviour is ‘adaptive to its bearers’ may wish to explore (as we did not) whether they are taking a position on the issue of whether a particular methodology makes it possible to determine what is adaptive and what is not in a rigorous way.Footnote ⁹

Our second reason for hesitation is that, putting methodology aside, we are now quite convinced that law professors (at least qua law professors) are in no position to determine when and why the pair-bond arose – period. Reaching that point nonetheless took us through a fascinating and time- consuming inquiry. No one without tenure should attempt it. Researchers speculate that the human species diverged from a common primate ancestor approximately 6–7 million years ago. No record exists, of course, that makes it possible to determine early human behaviour, so anatomical remains provide the best clues. Biologists hypothesise that ‘the more mates a male can expect to have in comparison with other males, the larger he is likely to be in comparison with females of the species’.Footnote ¹⁰ A high degree of sexual dimorphism is therefore inconsistent with the existence of a sexually exclusive pair-bond. Some of the earliest hominid remains are those of Australopithecus afarensis, a species that lived approximately three and a half million years ago. Initial investigation of the bones concluded that the species exhibited a high degree of sexual dimorphism. More recent publications suggest that males and females exhibited no greater differences than modern humans.Footnote ¹¹

The stakes in this research are enormous – at least for scientists and social scientists. Some researchers maintain that the pair-bond arose soon after the human line diverged from other primates, and is therefore an important element driving the evolution of the species. Others contend that the pair-bond did not arise until much later in human development. Bi-pedalism, and not the pair-bond or larger brains, then becomes the more critical evolutionary development.Footnote ¹² Still others argue that the pair-bond arose together with greater human intelligence, but that both occurred a long time after the divergence from other primates. To the extent that the pair-bond and greater intelligence occurred at the same time, they may have influenced each other.

This debate is enthralling, in part, because it addresses the intersection of gender and the rise of human intelligence. The original speculation about the origins of human intelligence focused on ‘man the hunter’. In this account, evolutionary biologists argued that ‘the transition from mere foraging to large-game hunting provided a major impetus for human evolution, with a cascading set of consequences including a rapid expansion of tool making and tool use, the development of a large human brain, and the evolution of complex language skills necessary for communication on cooperative hunts’.Footnote ¹³ Feminists objected that this made women no more than passive beneficiaries of the increased importance of male activities, and they countered with scenarios that focused attention on the evolution of gathering and co-operative patterns of child care. The most recent theories substitute emphasis on mate selection. Darwin had demonstrated in his initial work on evolution that some developments (e.g., the peacock’s tail) could better be explained in accordance with their attractiveness to potential mates than in terms of their advantages for survival. With the rise of the pair-bond, humans may have become more inclined to look for witty, empathetic or entertaining mates, conferring a competitive reproductive advantage on the more intelligent or verbally adept. The pair-bond itself would then explain the rapid rise of human intelligence and dominion over other species.

Indeed, the successive waves of information that we – and the scientists upon whom we relied – discovered helped reinforce the significance of who is asking the questions, and how are they framing them before drawing any conclusions on how the law could be affected. The grandmother hypothesis provides another fascinating example of this: why do women, unlike men, live much longer than they can reproduce? Feminist anthropologists have found female gathering, not male hunting, often contributes the majority of the groups’ sustenance in hunter-gatherer societies, with 60–80% of some groups’ calories coming through women’s gathering of food from plants.Footnote ¹⁴ Given that hunting produced food at uneven intervals, and societies typically functioned at a subsistence level, women’s gathering provided a more consistent source of food, and men may have needed women for their own survival.Footnote ¹⁵ Other researchers have suggested that scavenging, by both men and women, contributed more than big game hunting. More recent research has added that among contemporary hunter-gathers the most successful hunters share their spoils with other men as much or more than with their mates or children.Footnote ¹⁶ And, in these tribes, mothers are more likely to turn to ‘alloparents’, that is, grandparents, aunts and cousins, than to their lovers for assistance with their newborns.Footnote ¹⁷ Indeed, for females, pair-bonding may not be in their evolutionary interest;Footnote ¹⁸ women in contemporary hunter-gatherer societies report that men may be a net drain on their resources by consuming food and requiring care. Kristen Hawkes suggests that these alternative explanations based on co-operation may even suggest that ‘more exclusive pair bonds may [have been] added to a set of older interdependencies. . .the assumption that nuclear families are units of common interest may hamper our explanations of classic patterns in ethnography as well as of ‘newly emerging’ family forms in the postindustrial world’.Footnote ¹⁹ Helping question the ‘naturalness’ of any forms of human behaviour itself may be a productive route for inquiry.

Which view is correct? We certainly do not know. Considering the possible role of the clitoris in distinguishing between committed mates and opportunistic cads may be a better way to spend professional time than contemplating the role of dissomaster in child support awards, but law professors are not trained and should not be expected to weigh in on the former. Indeed, it is unlikely that any researcher today can determine the rise of the pair-bond with confidence. And if no answers exist, then it is not possible to identify the degree to which the pair-bond ‘was adaptive in past environments’. While we suspect and most evolutionary biologists maintain that the pair-bond is likely to have conferred a reproductive advantage, we certainly cannot say that the advantage arises from the meat provided, the assistance with the child, the protection of the mother from possible dangers, or the advantages that come with in-laws.Footnote ²⁰ For law professors, productive inquiry must lie elsewhere.

II. Modern biology and human behaviour: a more fruitful inquiry?

Our hypothesis about human mating practices, of course, did not originate with observations of the Pleistocene. In similar fashion, we have found summarising the evidence from contemporary biology texts more productive than attempting to evaluate the finds from archaeological digs. Once we let go of the efforts to understand the origins of the species, we found two types of less controversial evidence that strongly supported our argument.

III. Historical, anthropological and sociological inquiry: should law professors apply?

The hominid line diverged from other primates more than five million years ago, erect hominids with modern features have been around for roughly a million years, and homo sapiens have been in Africa for more than a hundred thousand years.Footnote ⁴⁸ ‘Civilisation’, in contrast, dates back no more than a few thousand years, and the history of civilisation could arguably be cast in terms of the effort to secure family regularity. For law professors who wish to test the hypotheses that emerge from evolutionary analyses against the recorded history of human institutions, establishing the origins of an evolutionary past may be less daunting than examining the extent to which it is possible to alter the resulting behaviour. For those who would test these hypotheses in the context of the family, two immediate obstacles arise: the literature is vast and the very effort to identify general patterns of behaviour may be in disrepute.

The history and anthropology of the family arguably start in with Frederick Engels’ argument in the late nineteenth century that monogamy followed the rise of agriculture and served to ensure the husband that all of his wife’s children were ‘his’.Footnote ⁴⁹ Yet, while Engels’ theories of patriarchy (and its association with monogamy) remain influential among feminists, he was wrong in most of his particulars. Moreover, these errors have discredited the effort to find universals about family development. He was certainly inaccurate in associating hunter-gatherers with wholesale promiscuity. He was wrong (and condescendingly moralistic) in his description of the family practices of non-European societies. And anthropologists have thoroughly rejected his claim of unilinear patterns of family evolution.Footnote ⁵⁰

What did replace Engels’ study of the family was much more particularised discussion of the practices of different societies at different times. On the one hand, this makes any comprehensive study of different cultures over different times daunting, if not impossible. General statements about the role of biology, or inquiries about the transition from hunter-gathers to modern arrangements become as much of a morass for law professors as explorations of sexual dimorphism during the Pleistocene. (Indeed, it took us almost as long to rework and mostly discard a lengthy digression on the family changes that accompanied the rise of agriculture as it did to determine that we would never reach closure on the evidence for the pair-bond in early hominids.)

Yet, just as we found when we abandoned archaeology for neuroscience, the interdisciplinary studies that focus on more particularised accounts of behaviour in our own culture provide a potentially rich and more relevant basis for considering the role of biology than overly generalised historical claims. More recent historians, for example, have refined Engels’ discussion of monogamy to transform it from a universal statement about the hallmarks of civilisation to an examination of the distinctive culture of northwestern Europe.Footnote ⁵¹ These records show illegitimacy rates in England and other parts of northwestern Europe to be extraordinarily low, rising in the eighteenth century from less than 1% to under 7% only with greater urbanisation.Footnote ⁵² Moreover, Edward Shorter’s extensive review of evidence ranging from birth records to rural doctors’ diaries similarly concludes that the level of sexual activity over several centuries of northwestern European history may have been remarkably low in comparison with either hunter-gatherer or modern sexual practices.Footnote ⁵³ These records, of course, cannot determine whether marital children were in fact the biological offspring of their mothers’ husbands, but the more general accounts of the policing of sexuality in these societies suggest that the rates of falsely identified paternity are likely to much lower than the 10–20% of children who have been found not to be biologically related to their legal fathers in some more recent studies.Footnote ⁵⁴ They suggest that the assumption that marriage, separation, divorce and adultery may not be as ‘insensitive to the imposition or legal prohibitions, or other costs’ as our assumptions about evolutionary history might suggest.Footnote ⁵⁵

Nonetheless, the comparisons with today’s society are startlingly. We have gone from non-marital birth rates of under 7% in the eighteenth century to 40 per cent of all U.S. births today. Divorce, which was virtually unknown in some time periods, has plateaued at one in every two marriages. The majority of American children may now spend some part of their childhood in a single parent family. While high death rates in earlier centuries make comparisons of divorce rates less exact, historians can document the exertion of considerably more energy (with greater likelihood of success) in the policing of sexual activity in earlier periods. Differences of this magnitude necessarily reflect the flexibility of human society rather than biologically mandated behaviour. Does a better understanding of biology have anything to add?

IV. What does biology contribute to legal research? The lightbulb goes on

In examining this kind of data, the fascinating parts are not the commonalities in human behaviour, but the variations. Some contemporary writers, convinced that the behaviour we observe in today’s societies reflect universals of human behaviour, simply reject the claims that historical times were different.Footnote ⁵⁶ If Stone and Shorter are accurate, however, the contribution of biology may be to help us focus on the factors that make societies regulate sexuality differently. To further this understanding the Law of Law’s Leverage, which we discussed at the beginning of this paper, may be helpful. For example, if romantic love is biologically primed to be fleeting, then laws that help love endure, such as mandatory periods of separation, as observant Jews prescribed for certain times of the month for women, or that make it more difficult to leave a marriage after the first two years may promote family stability. Speculation as to why there are different stages of ‘love’ is useful and interesting, but we also must acknowledge that the science is still in its infancy. We began our inquiry into the evolutionary basis of family stability, after all, not to resolve scientific disputes about human biology, but to critique family law. Family law in the U.S. and elsewhere has historically served to reinforce the values on which family stability rests. Can biology help to make sense of the transformation of the modern family that is now taking place and of the possibilities for guiding it?

The secret, we believe, lies not with developing a new, biologically based model of sex, marriage, gender differences, divorce or reproduction, but of revisiting our existing models. The conventional account of family change, to which both of us have contributed, ties the reorganisation of the family to the changes wrought by the post-industrial economy.Footnote ⁵⁷ To oversimplify: the older Anglo-American system linked family wealth, first, to land ownership, then to the husband’s workforce participation. Within this system, women’s chastity was a closely guarded commodity,Footnote ⁵⁸ and marriage occurred at or relatively soon after the beginning of women’s sexual activity. Earlier societies then placed the wife under her husband’s supervision as both contributed to economic production within the home. The nineteenth century remade that bargain for the American middle class, as increased male specialisation rewarded greater investment in children, commercial production increasingly occurred outside of the home or farm, and women’s roles were redefined in terms of primary emphasis on children. Financial interdependence and social stigma made it difficult for women to leave improvident relationships, and men were imbued with responsibility for their families’ financial well-being while the double standard sometimes permitted some leeway for male affairs that did not threaten family stability.Footnote ⁵⁹

If nineteenth-century changes turned on increasing specialisation among men and greater distinctions between home and market, then the twentieth century’s post-industrial economy brought with it increasing specialisation among women and dissolution of the distinctions between home and market. Demand increased for women’s market services, and rewarded greater investment in women. Women then reorganised themselves, as higher earning women hired other women (and McDonald’s and frozen foods) to provide the less specialised services they had once performed within the home. As women joined men in securing higher education, the average age of marriage increased. As women married later and their income rose, they also became less dependent on men and marriage, and as the consequences of divorce and improvident pregnancies became less devastating, men felt less responsible for the negative consequences that did occur. The economic coercion that complemented the moral suasion of the older family system gave way, and so did the effectiveness of the mechanisms that had promoted family stability for centuries.Footnote ⁶⁰

What can biology add to this analysis? We have concluded that it may shed indispensable light on the evaluation of the possible alternatives. Adding biology to our older analysis has led us to a new thesis: managing attachment in the modern era requires a reexamination of the interaction between family formation and different stages in the adult life cycle. The result is a research agenda that we hope will bring new insight, not into evolutionary analysis, but into what Justice Scalia has termed ‘the culture wars’. The thesis that has emerged from our biological inquiries is that our story of the contemporary reorganisation of family life is only half-right. Instead, it appears that states have adopted different strategies to respond to the impact of the post-industrial world. ‘Blue states’ have adopted a new family strategy that places a premium on postponing childrearing until the peak periods of testosterone production have passed, and couples can be expected to form stable relationships with a minimum of external coercion. ‘Red states’, in contrast, continue to link family stability to the control of sexuality, and internalised values that associate sexuality exclusively with marriage and reproduction.Footnote ⁶¹ They do so in the context of more traditional, often more rural, and more working class cultures in which adults are ready to begin childbearing at younger ages, and expect to marry shortly after women become sexually active. The Bible Belt and Utah – the reddest of red states – also have the highest rates of divorce in part because the degree of coercion necessary to keep couples together increases inversely with the age at which they begin marriage. The result is an irreconcilable clash between two very different moral and family systems.

Our new thesis takes us into a more discrete examination of biology. We wish, first, to examine more thoroughly the biological changes that occur in early adulthood and the impact of the different legal strategies on the manifestation of these changes. An extensive literature ties risk-taking, crime, adultery and divorce to age. Testosterone levels and brain chemistry are the expected causal agents, and we wish to take a closer look at the strength of the studies that support these assertions. Evolutionary analysis, however, is ready to supply an explanation: testosterone levels may peak during the prime periods for childbearing, and if so, they may suggest a tradeoff between ease of conception and relationship stability.

Second, we have completed a preliminary examination of the relationship between physical maturity, marriage and child-bearing. We have found that, in 1840, the average age of menarche was 16Footnote ⁶² and the average age of marriage for women was 19. In the U.S. today, based on better nutrition and health, the average age of menstruation has fallen to 12,Footnote ⁶³ while the average age of marriage is now well over a decade later.Footnote ⁶⁴ We believe that it will be relatively easy to maintain that at least part of the reason for the blue state shift is the impossibility of containing female sexuality for a decade or more while ‘unsupervised’ women complete their education and enter the workforce.Footnote ⁶⁵

Third, what role does jealousy play in the new system? Does the availability of effective contraception simply lower fears about paternity, or does jealousy affect the patterns in ways we have not anticipated?

Fourth, we would like to consider the evidence for the permanent impact of upbringing on brain patterns. If we were able to conduct our own MRI experiments, we would propose comparing the responses of those raised in repressive sexual environments with those brought up in more liberal surroundings. Do modern researchers have a different view of what Freud discovered when he tried to analysed the repressive childrearing practices of central Europe?

These inquiries, however, are tangential to what we expect to be the main subject of our research: documenting the divergence of family law systems that still attempt to pass judgment on sexual activity from those that do not. This examination involves the traditional methodology of the legal academy – looking at statutes and cases – and some relatively simply statistical analysis. We believe that such a study will show a correlation between lower ages of marriage, higher levels of divorce and stricter efforts to limit access to abortion, deny recognition to same-sex relationships, and consider non-marital cohabitation as a significant negative factor in custody awards. In the process, we expect to consider whether other demographic variables, such as religious observance, race, class and income, independently affect the outcomes. We believe, however, that if we find the correlations we expect to find, a significant difficulty with the research will be a chicken-and-egg problem: do states have younger averages ages of marriage because of the efforts to control sexuality, or do the efforts to control sexuality lower the age of marriage from what it might otherwise be?

As law professors, however, we believe that the research will have been worthwhile if we can simply show that two internally coherent and incompatible family law systems are emerging. Establishing a link to age of marriage will be a significant plus, but identification of the implications of two such systems is the traditional stuff of legal analysis. If we had arrived at the thesis from the more conventional analysis described above, our interdisciplinary inquiries might have focused in greater depth on economic change. With this agenda, we expect to focus to a greater degree on demographic differences, and the implications of divergent rather than converging normative systems. Biological observations may not be determinative, but they have the potential to fundamentally reframe the inquiry.

Conclusion: is it worth it?

We remain excited by this research despite the dead ends we have encountered. Nonetheless, we do have doubts about recommending it to others. Some of us believe that law and economics, at least in its early stages, became so popular because the barriers to entry were low, and the results easily supported the predetermined positions of some of the proponents. The result was to change the parameters of discussion in ways that enhanced the status of some, and skewed the resulting outcomes at least until the rest of the academy caught up.

Evolutionary analysis of law risks a similar fate, but there are nonetheless significant differences. If evolutionary analysis simply encourages speculation about evolutionary origins, speculation, for example, that men are more likely to commit adultery than women,Footnote ⁶⁶ the barriers to entry will be low, but the proponents are likely to encounter the charged opposition that now faces proponents of the older generation of law and economics. Instead, we believe that the integration of this type of analysis into legal scholarship involves the steps we have tried to describe here.

First, tying the origins of human behaviour to hunter-gather reproductive advantages is more likely to inspire speculation and amusement than satisfying resolutions, particularly for the law.

Second, overly general statements like ours, i.e., that homo sapiens are primed for pair-bonding, but not long-term fidelity, are not likely to be terribly useful, even if true. Either they cannot be proven or they restate the obvious.

Third, more nuanced statements (e.g., couples who marry as testosterone levels begin to wane are more likely to stay together) need to be empirically tested even if they correspond to evolutionary theories about reproductive advantage.

Fourth, evolutionary analysis will become persuasive as it is integrated with biological studies that identify the causal mechanisms (e.g., testosterone) that more immediately influence human behaviour.Footnote ⁶⁷

Fifth, biological insights do not stand alone; they must be integrated with more traditional legal and/or sociological analysis.

In the end, however, what these insights do most effectively is to change the questions that are asked and the framework used to evaluate the answers. We are convinced that we could explore the differences between family law systems that mediate childbearing at 19 and at 29 without reference to biology at all, and that most of our legal and normative discussion would be the same. But we are not sure that, without the lengthy inquiry described in this paper, we would have ever focused on the questions of age, and we are certain that we would not have framed our inquiry in the same way.