What Makes Us Human? It’s All about Sex! Part 1

One of the biggest questions in evolution, and one of the biggest challenges thrown at materialists by believers in a creator, is what makes human’s different from other animals and how can these be explained by natural processes? Over the next two posts I will discuss the works of Geoffrey Miller, who has come up with an overwhelmingly strong answer. Here I give an overview of his first book The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature where he sets out the theory of how we have come to be what we are.

In the follow up I will discuss his second book Must-Have, which explores what this means for modern society and I will look at what these theories mean for my philosophy of Spiritual Materialism. If I have done Miller’s central tenets any justice in my two unfairly brief summaries you will hopefully be as convinced of their merits as very strong scientific theories for their ability to explain so much of what we see, without conflicting with what we know, about human psychology and evolution, and making a vast number of testable predictions. Unfortunately I don’t have the space to present the evidence Miller does for his proposals, you will have to refer to the original works for that.

(There is also a healthy amount of educated speculation based on what we do know, both from myself and Miller, this is included to spark discussion rather than because I am convinced of their certainty.)

Miller’s first wide reaching theory sets out to explain what we see as our uniquely human mental traits (and one or two physical ones too) and thereby very thoroughly covers almost all male and female desires, intentions, interactions and differences. The crux of his theory is that those uniquely human mental qualities; academic intelligence, reasoning, artistic abilities, empathy, humour and ideology, may have developed through evolution of sexually selected traits.

What defines a sexually selected trait? They tend to be unique to a particular species, tend to be very different from closely related species (think about the unique colouring of many birds), but with a large variation between individuals of the species and they also have to be highly heritable. This matches those mental qualities, just like all sexual traits intelligence, wit, religiosity and aesthetic preferences are highly variable and we now know that many of them are highly heritable too. IQ, personality and interest in religion are known to be partly genetic. As someone training in evolutionary biology (I’m currently finishing my PhD in the area) that combination of a trait of being variable within a species yet little parallel in closely related ones is such a clear sign the trait is sexually selected I’m surprised it hasn’t been suggested before, Miller covers some of the historical resistance to the idea of sexual selection in humans in his book (it has a lot to do with Victorian, and more modern, gentlemen not sitting comfortably with the idea that women influence mate choice, as implied by Darwin’s second great theory).

Sexually selected traits evolve due to their increasing the number of chances for an animal to mate, rather than increasing their survival prospects (the latter of which is only important anyway because increased lifespan leads to more matings) and both are only important as far the passing on of those genes that helped the animal survive and reproduce is concerned. The difference between sexual and natural selection is subtle, and some evolutionary biologists don’t like to distinguish between them (both are of course ‘natural’, for instance).

There are two main theories as to how sexual selection works, i.e. why it occurs and persists in the context of passing on genes in an evolutionary process. The first is the classic runaway process; sexual traits are enticing to members of the opposite sex and they choose them merely because they are stimulating and irresistible. However, this means the genes for those traits gets passed on and the chooser’s offspring also have those traits, so there is an advantage over evolutionary time as more and more of a population gets better at increasing their number of chances to mate. This is usually called the ‘sexy son’ hypothesis because usually the males have the desirable, stimulating trait, such as the male peacock tail, and so by choosing the male with the best tail the female will produce her own son who have inherited a sexy tail. Not only that but the female must have gene for a preference for a quality tail and her daughters will share this indulgence. Thus the male tail and the female preference ‘runaway’ with each other and exacerbate one another over the generations.

The second theory of sexual selection is called the ‘good genes’ hypothesis and suggests traits are chosen by members of the opposite sex, which indirectly indicate fitness, again the peacock’s tail is an example, because his tail is large and cumbersome requiring a lot of energy to maintain in good condition, free of parasites and wear and tear, and a lot of strength and speed (and perhaps ingenuity) to escape predators when handicapped by a heavy tail that makes them more noticeable, they require a lot of resources, which would handicap an individual with low fitness. Therefore a peacock with a high quality tail is a proxy for good genes. Consider that the human brain is costly, 2% of our body mass uses 25% of our energy consumption, this doesn’t make sense in light of natural selection’s culling of wastefulness. However, in sexual selection, the best fitness indicators are wasteful; body ornaments with no practical survival use or behaviours that waste time, which could be spent on more essential survival tasks, like finding food. In fact, it is theorised that it is this very wastefulness and an individual’s survival in spite of a sexual ornament’s drain on resources, that makes the ornament attractive; if you can survive despite the encumbrance of an ornament then you must be an able partner. In humans Miller argues that most of our ornaments are cognitive. This means that we have devised ways to show them off and ways to judge what is shown off through sexual selection. So someone who can demonstrate good running of their brain, through showing off their learning, dedication to a belief, rhetorical and verbal skills or creative expression are indicating that they are fit enough not just to survive, but to feed their costly brain to excel in the areas that the chooser prefers. Note of course that neither peahens nor people are aware we choose are partners because they are indicators of good genes, simply that genes that are good at judging will be passed on with the good genes and the indicators and produce offspring that are good at attracting mates and surviving.

Which of these two theories is the ‘correct’ one is not important for Miller’s argument (the consensus seems to be heading towards sexy son), they are not mutually exclusive and both may apply to humans. You may have noticed that sexual selection also tends to be applied more often to sexually dimorphous traits, those that are very difference between males and females, like the peacock’s plumage compared to the peahen’s. However, this is not always the case, sexual dimorphism is not a rule for sexually selected traits like the other things noted above, not only because dimorphism often relates to more practical elements of reproduction and care of offspring but also because some traits that are equal between the sexes and still sexually selected. Although Miller discusses some physical traits that are sexually dimorphous in humans (which I won’t go into here), there is little to no gender difference in human mental abilities of the sort I’m discussing here, I discuss why in the next section.

The Sexy Brain

Miller’s major proposal in The Mating Mind is that in humans fitness indicators of good genes or our enticements to members of the opposite sex are our unique mental traits, intelligence, humour, morality, that act as advertisements of our ability to maintain a costly brain and/or how stimulating and irresistible we are.

These advertisements are signals which aim to increase mating opportunities. This does not necessarily mean to increase chances to mate with as many people as possible, but can mean increase the number of people who might take interest to increase the pool of potential mates to choose from and be chosen by. Like all animals humans try to balance the opposite strategies of partnering briefly with as many as possible (high quantity, low investment) and putting a lot of effort into one partner and their offspring (low quantity, high investment) with greater influence their on quality and survival. To what degree we use each strategy is still hotly debated in evolutionary biology, usually said to be a mix of both strategies with great variation between individuals and cultures and between the same individual at different times and in different circumstances. Although the consensus perhaps comes down on the high investment side considering how much the long infancy of humans require, Miller actually leans somewhat against this, arguing long term monogamy is unusual is our evolutionary history and, since it takes on average three months of intercourse for humans to conceive, relationships of around this length would have been the most common.

Humans take on average three months of sexual intercourse to successful conceive a child, this is extraordinarily long compared to any other animal species. Miller argues this was an adaptation that led to, and/or was a result of, females selecting males that would keep them interested and stimulated for at least that duration. After which they then have a pressure to impress the male enough for him to stay around. Males, on the other hand, after originally choosing a woman to try his luck with (although this is the other way around in some cultures with females choosing and approaching initially), has pressure to impress her up to and during intercourse and repeatedly for the first three months afterwards. Therefore, women are making mate choice decisions before intercourse and in the first three months, men make them after that; they become pickier as they decide whether the woman is worth investment to stay around in the future. Males in many animal species have to make the decision whether to stay with a female who will produce high quality young that are worth looking after to ensure their potential is met, or whether to look elsewhere, not just in order to go for quantity over quality, but simply to find higher quality. Hence why many women experience that their partners become quieter, less thoughtful and less helpful after the first few months or years as the man may have no impetus to impress with his expressions any more, he doesn’t feel compelled to display. Therefore, we see that both males and females both choose and display in human sexual selection.

There is some difference in the sexes when it comes to choosing and displaying, the fact that males have to display earlier and more quickly means they have to be more overt and ‘aggressive’ in their signalling. This could explain why the majority of publicly known poets, painters, musicians, stand-up comedians and even scientists are males. That’s not to say women don’t or can’t do these things, in fact research shows time and time again, men and women are equal on these traits, women simply don’t make such a song and dance (sometimes literally) about their abilities and achievements. (Nor is it meant to deny the glass ceiling, part of the problem may be that women aren’t as inclined to self-promotion or it may be unrelated, to draw a moral lesson from an attempt to explain something scientifically is the naturalistic fallacy.) As for whether we would expect any differences in capabilities between the sexes, well if men have evolved to court with humour, aesthetics, knowledge and ideology, women have need to have evolved to judge. Humour can only be judged with a sense of humour, aesthetics with a sense of beauty and knowledge and ideology with logic, intelligence and emotions that can be deeply moved. Men and women are in a constant arms-race to develop better mental quality indicators and better mental quality detectors and this has pushed them to being equal on average at these traits.

On this theory all expressions of experience, through art, music, conversation, canvassing, perhaps even the very experience of the self, have unconsciously developed to advertise the hidden traits of mental fitness (and the physical fitness that allow upkeep of our brain) to potential mates and rivals. What follows is an assessment of how some of the central parts of human life are affected by displaying and judging the mating mind.

Ideology & Reality

If non-sexual selection was the only force at play we would expect evolution to produce species with brains that visualise and interpret the world with ever greater accuracy and precision. However, the mating mind would have favoured beliefs about the world that are selected for other values; moral, dramatic, humorous or inventive. Our recollection and interpretation of events, then, is set up to show us in an attractive light, not in a realistic one.

“Sexual selection usually behaves like an insanely greedy tabloid newspaper editor who deletes all news and leaves only advertisements. In human evolution, it is as if the editor suddenly recognized a niche market for news in a few big-brained readers. She told all her reporters she wanted wall-to-wall news, but she never bothered to set up a fact-checking department. Human ideology is the result: a tabloid concoction of religious conviction, political idealism, urban myth, tribal myth, wishful thinking, memorable anecdote, and pseudo-science.”

In my next post I will explore what this means for society and the development of a spiritual materialism.

Romantic Religion or Sexy Science?

Some of the factors most strongly correlated with higher religiosity include gender (being female), wealth (lower) and education (lower, although the relationship is complicated). Those who are married without children or never married also make up most of US congregations, which fits with the time before having children as being one of signalling and choosing (although there are of course other possible reasons, it’s harder to find time for church with children). Callous as it may sound, could it be that the poor and uneducated don’t have fitness indicators that others have, money, (academic) intelligence/qualifications, so they use religious belief and worship? Women are much more likely to be religious than men, Miller makes no mention of this, but I wonder about many possible conclusions that can be drawn from this fact when refracted through the prism of the mating mind. Does religion provide a system of morality and dedication through which women can judge men? Miller does entertain the idea in his next book that women have more incentive to show they are religious as they have more to lose by being rejected from their family or community. Men can more easily weather the effects of controversial views, and may even get benefits from having them. Certainly it is men who tend to be theologians, people preoccupied with showing off their intelligence in working out how their beliefs can seem logical. Men also in general identify more openly as atheists and are more outspoken in giving their arguments for their conviction.

Worship fits the expectation of the best fitness indicators; they are wasteful in order to show that individual is fit enough to waste time and resources. This is no problem for a spiritual materialism that says the beliefs can be jettisoned and the practices kept, but it adds a complication to my argument that only those practices demonstrated by science to improve human mental and physical well-being should be maintained. If practices are an indicator of the mating mind traits, then they have other uses. However, there are many other ways to show off our mating mind, and I have no problem with the scientifically selected practices developing extra trappings. ‘Scientifically selected’ does not need to mean devoid of creativity and aesthetic embellishment as long as the changes made do no physical or mental harm to others.

Religious faith can also be seen as an aspect of the mating mind. Showing that you have the dedication and imagination to attest to something that there is no evidence for, and furthermore creatively interpret the imagined being’s intentions and change ones behaviour for it, could show all kinds of fitness indicator. The brain power needed for the theological contortions around contradictory evidence is quite impressive even to an atheist! Are women more likely to be religious because that is their sexual selection preference? They then put the onus on men to demonstrate their religious fitness indicators, who, perhaps lacking other indicators such as wealth or education to show off, are more willing. Of course in the past no-one was wealthy or educated, once begun the parts of the brain involved for religious emotions, experiences and general beliefs could be selected for. Some of this may explain the putative adaptations that make believers consistently reporting higher levels of happiness, which we know is related to social factors.

The conflict between religion and science nowadays, one is tempted to compare to the antagonism between natural and sexual selection, that of survival versus reproduction. If we take the purely pragmatic view of science, that it is meant to be useful, to find out about the universe so that we can harness it to increase food output, comfort etc, then that is in conflict with any opposing ideology. And ideology is a form of signalling in sexual selection. Of course the additional justification for science, that knowledge is worthwhile for its own sake, is in this view an ideology, as knowledge can be used to signal mental fitness and so defence of its inherent worth is also a signalling strategy, it is a vanity project, albeit one that happens to be constructive to others once science bears fruit.

However, as Miller points out,

“From a sexual selection perspective, science is a set of social institutions for channelling our sexually selected instincts for ideological display in certain directions according to strict rules… Science is not asexual or passionless. But neither is it a result of some crudely sublimated sex drive. Rather, it is one of our most sophisticated arenas for human courtship, which is the most complex and conscious form of mating that has ever evolved on our planet.”

Again, what hope we have as to whether we can overcome the allure of religion as a fitness indicator or whether we should resign ourselves to the fact that it is destined to stay, will be addressed in my next post.

Art, Poetry and the Self

Art is sometimes seen as the most pure, transcendent or divine from of expression of experience, but it is still a signal (or series of signals) about mental fitness. Therefore we can answer the age old question ‘what is art?’. All arts are vanity projects and art is whatever one can convince others is art, since convincing them it is art means one has impressed them with how one expresses experience, and impressing others increases reputation and mating opportunities.

It has been theorised that the idea or feeling of ‘self’ is something that evolved for its survival benefits under natural selection, once we were conscious beings we are happier if we maintain an illusion of self to bring together the disconnected array of neural processes that is the human mind. Happier people live healthier, longer lives. However, it is suggested in The Mating Mind that self image may only be a construct to attract others, a cogent, complete self serves more the psychology needs of others than our, well, selves. An image is something projected for people to see. We most care about a consistent, logical self-image when it is questioned by others, ‘why did you do that when I told you not to?’ ‘Aren’t you forgetting you said the exact opposite the other day?’ And we will pull of impressive feats of politicking, often so easily and quickly that we don’t notice we are doing it, to save face. Or rather save a coherent self-image.


As with most discoveries pertaining to human mental life, especially from those believers who challenge the materialists to explain them without resorting to a creator in the first place, the charge of reductionism of the human soul will be made. I leave with a quote from The Mating Mind pointing out fact that knowing what makes us human isn’t going to make us any less human:

“Understanding the origins of human morality, art, and language is unlikely to diminish our appreciation of ethical leadership, aesthetic beauty, or witty conversation. On the contrary, if these human capacities evolved through sexual choice, then our appreciation of them, depending on a relatively hardwired set of sexual preferences, should be immune to any of the alleged wonder-reducing effects of scientific explanation. In any case, I trust that the enjoyment of worldly delights is better accompanied by true understanding than by romantic obscurantism.

– The Spiritual Materialist


6 thoughts on “What Makes Us Human? It’s All about Sex! Part 1

  1. Pingback: Religion, Sex & Personality | The Spiritual Materialist

  2. Something has occurred to me since writing this post. If it’s true that most of our ‘human intelligence’ evolved to adapt to mate choice pressures rather than environmental pressures, i.e. by sexual rather than natural selection, then this makes the chance of other life on this planet or others with human-like intelligence much less likely. A similar combination of approximate physical factors to give rise to the same environmental pressures is vastly more likely than an organism that would have to start off almost the same mental set-up, i.e. with certain mental traits, like the big 5, and a way to convey and show off those mental traits, like complex language, giving rise to the same mate choice pressures.
    For instance, it’s highly likely that organisms will need to escape predators, break their fall from a height or find widely dispersed food, so it’s highly likely that evolution will favour the development of wings, which has happened independently in birds, mammals and insects. In contrast, the coincidence of factors needed for sexual selection to favour the development of the ‘holographic’ eye designs on a male peacock’s feathers is vanishingly small and has only happened once.
    Yes, there are other birds with eye-like designs on their feathers and there are other animals with various degrees of intelligence, but in none are quite the same. How similar animal or alien intelligence has to be to human intelligence for us to communicate with them is still a big, unresolved question. But if we are talking about aliens that have technology they would have to have intelligence similar to us (as well as something like hands to manipulate with). Something to ponder anyway.


  3. Pingback: Science & Values: Is My View of Religion Reductionism or Scientism? | The Spiritual Materialist

  4. Pingback: Religion, Politics & Personality: It’s All about Sex! Part 2 | The Spiritual Materialist

  5. Pingback: Study: Religious Belief Acts as a Cue for Open-mindedness when Choosing Romantic Partners | The Spiritual Materialist

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