Understanding Sex Differences in Humans: What do we learn from nature?

Nature is a potential source of guidance for our behavior, morals, ethics, and other more mundane decisions such as how to build an airplane and what to eat for breakfast. When it comes to airplanes, you'd better be a servant to the rules of nature or the airplane will go splat. When it comes to breakfast, it has been shown that knowing about our evolutionary history can at times be a more efficacious guide to good nutrition than the research employed by the FDA, but you can live without this approach. Nature works when it comes to behavior too, but there are consequences. You probably would not like the consequences.

The question at hand is this: Should men and women be given fundamentally different rights? Would it be OK if men and women had different pay for the same job, or different access to jobs? Would it be OK if men and women were treated differently by the law in a way that accounted for the behavioral differences between them that arise from their biology which, in turn, may be partly a function of their evolutionary history? Should men and women have different status because of their gender? Similar questions can be extended to people that are biologically different in other ways, such as by age, gender orientation, physical handicap or, should it be proven a valid categorization, race. But for now, let's stick with the basic adult male vs. female difference.

[This is a heavily rewritten post originally published here.]

The idea is very simple: That which we observe in nature is the best guide to how things should be. We see that in mammals mothers nurse their young. Departures from this (bottle feeding, early weening, feeding young something other than mother's milk, etc.) are risky and often have negative consequences. In the modern, Western, industrialized world, there is a socially constructed balance between natural and non natural choices. A child that is fatally allergic to mother's milk would be left to die if being raised in a "state of nature." But in practice, the life of such a child is placed at a higher value than one's philosophical purity, and non-natural intervention (feeding the child soy milk from a bottle) is chosen as the 'correct' decision. In truth, day to day, we may be utterly arbitrary in adherence to or ignorance (willful or otherwise) of the naturalistic premise. We do what is convenient, what feels good, what provides us some good (money, status, etc.). Then later we explain our decision rhetorically as necessary. But that, dear reader, is a whole other post.

Nature, as a guideline, is often invoked when considering political or economic decisions. Free market capitalism is natural. Social Darwinism, survival of the fittest, is natural.

Let's look at the idea that nature argues for differential pay between men and women. The premise is that women get paid less than men. There is plenty of room for clarification here ... do women get paid less than men for the same exact job? Do women get paid the same but end up with a lower salary because they take unpaid leave to have babies? Do women get paid the same but end up with lower pay because they take unpaid leave which indirectly contributes to slower (in calendar time) advancement on the pay scale? Are women kept out of jobs, or even entire professions, that tend to be higher paid? Some or all of the above? For the present purposes, none of these questions matter, as you will see.

To orient the argument, let's consider the following list of hypothetical comments that may be found on the internet.

• Is every way we treat the two genders differently insulting? Why stop at 24% lower salary? How about holding the door for the weaker sex? How about only women getting to improve their daily look with make-up, while men doing it are ridiculed? Why must the stronger sex always carry all the groceries?
• Is paying men and women equally really fair? Women and men are different, have different strengths and advantages, and different limitations. Those are obviously a very large part of the reason why salaries are skewed.
• ...it is evolutionarily more important for men to earn money, as money is earned for status, and not for consumption.
• ...physically ... Men are stronger, taller, and don't get pregnant.
• Psychologically ... Men are more aggressive, more ambitious, more authoritative, more psychopathic, less caring of others ...
• ...being more aggressive, more ambitious, more authoritative, more psychopathic, less caring of others are "qualities" that are sought in CEOs...
• ...hiring a woman in a job involves the risk that she will be unable to work if she gets pregnant. The "worth" of that employee is thus modified as a result.
• ...if you hire a person who is likely to die soon the employee is worth less to an employer than someone who is guaranteed to live for a long time and work in that job.
• ...in divorces it is usually the wife who gets the children. .... The higher salary of men as compensation for that fact.
• Bottom line is salary difference has a biological basis. Until it is thoroughly understood why there is that difference why come out and say it should be abandoned.
• Women are on average less strong than men. That there is variation doesn't change that the probability that a random man is stronger than a random woman is above fifty percent.

I would argue that these and similar beliefs are not matters of opinion nor are they matters of political correctness. The discussion at hand has a deep and rich intellectual history, and embracing pure and unadulterated reference to nature in such a male-biased way (or any way for that matter) is no more acceptable than embracing a heliocentric universe as a student of physical sciences. We've been there, done that, and we called it the Middle Ages. Nonetheless, lets look at the argument in more detail.

A nature-based justification of human behavior may take into account the fact that we are mammals. Our mammalness encompasses many of the critically important facets of our lives. We have approximately two sexes, a male (producing sperm) and a female (producing ova). Pregnancy lasts a long time relative to the overall life cycle of a given female. The females nurse the young, adding significant time in the form of child care. In mammals, males fight or display for sexual access, and females are either herded or harassed by males or choose males with which to mate, and males provide virtually no offspring care in most species. In some species there is courting and female choice, in others, hormonally mediated sexual arousal and activity, in others, what we might call rape, or to chose a better term, forced copulation, may be routine.

That is a pretty wide range of behaviors, but one must use this wide range to describe 'typical' mammals, as they do vary somewhat. There are key characteristics that do pertain to all mammals, however: Pregnancy and nursing being entirely female, longish period of offspring care, and internal fertilization which results in a certain amount of paternal uncertainty (unclear attribution of fatherhood) for all males.

Given this, we may expect human males to be less choosy (sexually) than females, we may expect males to be promiscuous, we may expect females to be more cautious, we may expect males to be show-offs and often more violent than females, and we may expect males to be bigger and stronger than females.

But really, we are mammals but we are also primates, which is a subset of mammals. Would it not be more appropriate to look to primates, rather than mammals, to understand our fundamental natures?

Well, most primates are either solitary or monogamous, with males and females not differing very much in size. Mating happens as a matter of female choice more than male fighting in most primate species. In many primate species, especially the polyandrous ones (where a single female has two or more male mates) there is a certain amount of male care of offspring, while in others, not so much. There is not a big difference in the danger level of males vs. females in most primates; predators are not choosy in this regard. So, our evolutionary heritage as primates actually looks quite different than if we look more broadly at mammals. Based on a primate-wide model, we might expect male humans to track females very carefully, be more or less at their service with respect to child care, and there should be very little difference between the sexes in who gets to use force or coercion for personal gain. Males and females would roughly share the job of protecting home and hearth (proverbially or otherwise). Males in many cases would not know if they are the father of a particular female's offspring, but they would remain devoted to the female and her young because the young are related in some way (the multiple males hooked up to individual females would typically be brothers or half brothers, for instance).

But really, while we are in fact primates, we are actually Old World Primates. If we remove the prosimians and the New World Primates from the mix, we get a different picture.

Looking more narrowly at the Old World Primates, we actually drop all of the polyandry and most of the monogamy. We now get a pretty large difference, on average, in body size of males vs. females, but male coercion is rarely a means of sexual interaction ... rather, females and males both engage in quite a bit of politics (these are smart animals) and these political interactions are mediated by quite a bit of biting and poking (within both males and females, but maybe more so in males). The result is often a parallel (male vs. female) set of hierarchies, and position in these hierarchies determines for males who gets to mate and for females who ends up most successfully raising offspring.

From this perhaps we can understand such human behaviors as guys getting together to do sports and gals getting together to shop and compete over makeup and shoes. Gossip, politics, personal status, etc. are all expectable pastimes or passions from such an Old World Primate ancestry.

But wait, the Old World Primates diversified a VERY long time ago. Maybe we should look at the subset of Old World Primates of which we are a part ... the apes.

The majority of ape species are monomorphic in body size (the males and females are the same size) and life-long pair bonding. Both males and females are physically equipped (strong bodies, big canines) to defend the territory and the young, and both take similar roles in this regard, though the females nurse the young so there is some difference in male vs. female role in offspring care. A considerable effort is put into care of offspring overall, and with setting them up in new territories, etc., and this sort of care involves the males at least as much as the females.

So we might expect humans, as apes, to be highly monogamous and for both sexes to put huge amounts of efforts into offspring ... somewhat different in style but with similar levels of effort for males vs. females.

But hold on a second there... we are apes, yes, and this characterizes the average ape because gibbons and siamangs are all apes. But we are great apes! The great apes constitutes a smaller taxonomic group. Maybe we should look at the great apes only and forget the gibbons and siamangs.

OK, when we do that, we are looking at orangs, gorillas, chimps, and bonobos. Orangs have a very high level of sexual dimorphism, are primarily vegetarian, and the most typical form of sexual interaction is either forced copulation ("rape") or females swooning over gigantic, and presumably very sexy, but rare, super males. All offspring care is female. In fact, the largest social group among these apes is the mother and offspring with a random male busy raping the female while the offspring hangs out on a nearby branch eating some wild figs. Gorillas also have a high level of dimorphism in body size, but live in large groups with the key group structure consisting of a silver back male and a harem of females who are totally devoted to and sexually monogamous with the male until a lone silver back starts to show up and kill the female's infant offspring now and then. When that happens, the females join the infanticidal male and abandoned their devoted and gentle silver back.

These two apes provide very different models, but are similar in that females are either raped or have their children killed (and they can stop that by joining the killer) and when push comes to shove, the enormously large males get to do all the pushing. This would suggest that humans get comfortable with a very male dominated society and that the females should just get in line. Fast.

But hold on, we are much much more closely related to the chimpanzees ... common chimp and bonobo ... than to these other apes. So let's look at their lifestyles.

Both groups have the unusual and interesting feature of adult and potentially sexually mature males and females living in the same group. When a female is in a state of ovulation, she also enters a state of estrus ... the visible display of ovulation. Some of the males may be forced to not mate with this female (forced by dominant males) but for the most part every male mates with such a female. Over time, all of the females go into estrus one or two at a time. So, over the course of a few years, every single male will eventually have potentially baby-making sex with every single female. This is done in the form of giant orgies in which only one female participates.

That is true for common chimps, but it is also true for bonobos, with an added twist. All the chimps have lots of what I will call erotic interaction all the time, including auto erotic. But for bonobos, there is the added feature of almost every possible gender and age combination of erotic interaction, and every combination of body part interaction. So a young female may provide oral sex to an older male. An older male may provide oral sex to a young male. Two adult females may engage in genital-genital rubbing. And so on and so forth. Young male chimps do not seem to have sex with their mothers. Otherwise, pretty much every combination happens.

So, given the chimp model, we should all be bisexual and disregard age of our sexual partners. Almost all baby making sex should involve a gang bang lasting several days. We should have strong male hierarchies and female hierarchies that determine, ultimately, who gets to be the father of each child (more or less) not by who has sex with whom, but by regulating exactly when in the ovulatory cycle intromissive sex with male orgasm happens. If we lean towards the common chimp model, all males should be dominant over all females. If we lean towards the bonobo model, all females should be dominant over all males.

So, that is the sum of our naturalistic models ... where they come from and how we might use them ... assuming that our evolutionary heritage, our phylogenetic framework, our Darwinian determinism, should provide us with the best naturalistic guidance.

But hold on a second. Humans are ape, yes, but we are also part of a subset of apes that diversified from a chimp-like ancestor millions of years ago. Roughly speaking, these were the "Austrlopiths." They were chimp like in size, probably dimorphic in body size, with some species being as dimorphic as chimps, others much more dimorphic, in the gorilla and orang range. None of the adults had impressive canines, they walked upright and had hands that were probably better at manipulating tools than are those of chimps. Their upright stance may have made the estrus signal of a sexual swelling impossible. They lived in woodlands and savanna environments, not dense forest. These characteristics suggest that they may have been a lot more like chimps than anything else, but there is an important difference: Some of these species, especially after about 2.5 million years ago, seemed to use stone tools, and they may have had slightly larger brains. Using stone tools, especially chipped stone tools, adds a complication. Stone tools require some degree of investment, to find and shape the raw material. In a purely chimp-like social system, this can not really happen because any investment by the average individual would be wasted when a dominant individual came along and took the stone tool(s) away to use them. If Australopiths of this later period, or their close relatives, used stone tools very often and relied on them, the social system must have involved the ability to "protect" this investment, to make contracts among individuals to not be so chimp-like and grabby all the time. This implies that there could also be social contracts among individuals that may have allowed a different system of mating and child raring, one that might involve more monogamy and more male parental care.

At some point in time, just under 2.0 million years ago, human ancestors changed dramatically from this forest-ape form. They got big, about doubling in body mass, which meant being able to garner much more food from the environment. Their brains doubled in size, which required not only much more food (the brain is a hungry organ) but also special kinds of food for youngsters with growing brains. Also, the difference in body size between males and females went way down, and the stone tools became much more sophisticated, indicating that they were sometimes made and used for weeks or months, not just expediently. This idea of a social contract involving both possessions and mates probably became very important. Is is possible that for the first time, mates pairs could exist in a social group with a number of sexually mature males and females without too much of that crazy chimpanzee and bonobo behavior. They may have been very human like. But note, they would have been human like in ways that were not Australopith-like, not chimp-like, not great ape-like, not ape-like, not Old World primate-like, not primate-like, and not mammal-like. Some of the most important things about the behavior of those early members of the genus Homo, maybe nearly all of their distinguishing characteristics, were unique in the mammal/primate/monkey/ape world.

To help understand this transition in evolutionary history, we might consider building an alternative model based on nature, by reference to something that is not even a mammal: Birds.

We might be mammals, but we act like birds. Like chimps, we exist in societies with multiple potentially sexually mature males and females. But we tend to pair bond (or nearly so) within this framework. In this sense, we are very different than our closest living mammal relatives (who, by the way, are relatively very distant in relationship compared to many other pairs of species!). We are not that closely related to birds, but if we look at a wide range of human societies who are known to live off the land ('preagricultural' groups, either in the present or ethnohistorically known), we see that human societies are often very close to bird societies. We have some kind of monogamy that occasionally develops into a bit of polyandry (like traditional Tibetan highland groups and the phalaropes (birds) of the arctic) or a bit of polygyny (like many cattle keeping groups or the oft-studied oft-cited red winged blackbirds and many other birds). But even in societies that do allow polygyny, most families are based on monogamy, though it is serial monogamy (like the vast majority of bird species including almost all song birds). Yet, when certain economic features ... like land (nesting sites) and professional or social milieu (territories) are essential to status and wealth, we have very long term monogamous systems in humans such as the immutable Christian Victorian marriage (or in birds the life long bonding of raptors). In all cases, there is a LOT of care invested in offspring, and males and females deliver similar levels ... and in some species very similar kinds ... of this care in birds. In humans, there is also considerable care in offspring but ... alas ... we are mammals so males can't nurse the young, and this starts a cascade of male-female differences. Perhaps females care for the young directly while the males busy themselves defending the territory.

Why, it is rather remarkable how birds map human variation in society in so many ways. But not all. Birds rarely live in tightly knit, spatially close groups of sexually active pairs. One example of this is nesting sea birds like gulls and terns. And for gulls and terns, the big risk with respect to producing offspring is not so much that your neighbor has slept with your mate. Rather, the risk is that your neighbor eats your babies when you are distracted. Happens all the time with those creatures.

Dear reader, if you are still with me (and I would understand if you've gotten bored or frustrated and gone away by now) then you can easily see this point: We have a rich supply of models from which we can draw nature-based conclusions, and these models can be used to 'justify' or explain almost anything.

A better question might be: What is the premise we choose, as a society, to be the basis of our ethical and moral codes, our laws, etc.? For many people, this premise is mutualism. We agree to equality of all individuals (with special exceptions). This equality does not mean individuals are identical. Indeed, there may be categorical differences among groups. Females do have babies, males do not. But equal rights are to be preserved.

This does not mean that the consideration of and reference to nature goes away. What it should mean is that nature-based models can not be used to justify systematic social, cultural, legal, economic, philosophical, or political inequalities. But they can be used, if used properly (and that is an academic, not political issue), to explain some things. In my opinion, we are very very far from being able to explain much with what we currently know, and certainly not at the pop psychology level of which so many seem so fond.

But I do want to make an attempt at a nature-based consideration of modern human society with respect to two realities. One, females have the babies and males do not, and two, males tend to be more violent and aggressive than females.

The fundamental reality of these propositions needs to be tested first. Do the females really have the babies, and what does this mean? Well, it is not so simple. For the most part, females do have the babies but with modern approaches it is possible and indeed quite common, and in some cases, necessary, for males to have much more input in offspring care in humans than one might otherwise predict from a purely nature-based model. For example ... and very few people know this, and learning this is your reward for sticking with me this far along in this post ... I personally fed my daughter for her entire nursing period. I held her, I gave her the milk, we stared into each other's eyes and bonded, the whole nine yards. Not her mother. Me. So, while the female clearly has a major biological commitment to the process, it is not as absolute as one might assume.

With respect to male violence and aggression: Margaret Mead was wrong but not totally wrong. Males are always, without exception, more violent and aggressive, on average (and bigger and stronger too) than the females when the comparison is made in the same society. Maybe a little, maybe a lot, and males do not have a monopoly on this sort of behavior. The absolute level of aggression and violence among both males and females is highly variable to the extent that there are societies with females who are more violent and aggressive than the males in other societies. Most importantly, the level of difference between males and females in a given society ... and especially the level of male control over females ... varies greatly. There are societies in which there is very little difference between males and females, and there are societies in which the difference is great. Americans: You live in a society where the difference is considerable, more than the average. That is not how it has to be.

So, with respect to our individual selfish Darwinian reproductive goals, our broader social (territorial, economic, etc.) goals, and our cultural fixations, babies and aggression are both important. Offspring are our Darwinian legacy; sons are guns; little girls grow up and give their parents more Darwins (a unit of fitness). Sexual access must be ensured and paternity managed. Territory must be held, resources protected. And so on.

The problem is that only the ladies can have the babies, and it mainly falls to the gents to be the tough guys. On top of this, when a woman has a child she may fall short in some other responsibilities such as carrying all the firewood and water and other physically demanding tasks (as occur in most societies where women do the vast majority of hard labor). For their part, this aggressiveness of males comes in handy for defending the group territory, but becomes a nuisance when male aggression turns to beating, raping, murdering, and threatening others, mainly women.

So how do we deal with this? Start out by admitting that we as a society owe women a great deal for being the baby bearers. It is hard, painful, and you can die doing it. But no. In our society, we take away a woman's rights because she is the baby bearer. She is paid less, and often her value is diminished.

..hiring a woman in a job involves the risk that she will be unable to work if she gets pregnant. The "worth" of that employee is thus modified as a result....

We also deal with this by admitting that aggressive male approaches are not necessarily a good thing. Yes, it may be true that "... men ... earn money ... for status, and not for consumption." But that would be because men are being assholes. If it is true that "...being more aggressive, more ambitious, more authoritative, more psychopathic, less caring of others are 'qualities' that are sought in CEOs.." then we have to stop doing that. We have to stop seeking and rewarding those qualities.

Compensation works both ways. We must compensate, as a society, for the burden of our evolutionary past as manifest differentially by gender. Our behavior is flexible, and thus it is incumbent on our society to attenuate violent leanings. Childbearing is fundamental and essential but cannot be totally outsourced by the women who do it. Punishing women for having this responsibility is exactly the opposite of what we should do.

A review of our evolutionary context is interesting to me (it is what my professional research life is entirely about) and this context is causative. But a realistic look at our evolutionary biology does not give any simple answers, and never, ever does it provide justification for unfairness or violence.

There is a reason they call it the Naturalistic Fallacy.