Discovering Human Nature
Helena Cronin

Introduction Helena Cronin is well known for discussing controversial issues, such as the marketability of designer genes and the ability to literally measure beauty. Cronin argues that Darwinian ideas can help us discover and understand the psychological and physiological differences between men and women. She uses evolutionary theories to develop an understanding of human nature--from beauty, sex, domestic murder and child psychology to our preferences for sugar, fat and salt.

In one of my favourite cartoons, a hopeful patient asks his doctor, "Have you got something for the human condition?" One cannot but sympathise with the hapless physician. Or so I used to feel. Now my urge is to leap into action, crying, "Let me through, I'm a Darwinian; perhaps I can help."

In the past decade or so evolutionary theory has made--almost literally--a mind-blowing discovery. It has pried open the neatly arrayed toolbox that is our mind, mapping its specialised components like an exploded technical drawing. Just as Gray's Anatomy lays bare the human frame, so Darwinian scientists are beginning to write the owner-occupier's manual to that hitherto most recondite of mysteries: human nature.

For, contrary to dogmatic denials, human nature exists and it is universal. Our minds and brains, just like our bodies, have been honed by natural selection to solve the problems faced by our ancestors over the past 2 million years. Just as every normal human hand has a precision-engineered opposable thumb for plucking, so every normal human mind enters the world bristling with highly specialised problem-solving equipment. These capacities come on stream during development as surely as the toddler's first faltering step or the adolescent's acne and ecstasy.

This mind-and-body-building is orchestrated by genes. But we're not automata, their slavish puppets. Certainly, genes can do their work single-mindedly; and we're thankful for it every time a perfectly formed baby is born. However, genes also underpin the flexibility and variety that typify human behaviour. The instinctive rules that underlie human language, for example, are precise, rigorous and universal; but they have enabled us to generate the multiplicity, richness and diversity of all the languages that have ever existed. It is because our mental instruments are designed to pick up cues from our ever-changing surroundings, process that information and decide on action that we behave appropriately, inventively and ingeniously.

All this apparent design has come about without a designer. No purpose, no goals, no blueprints. Natural selection is simply about genes replicating themselves down the generations. Genes that build bodies and do what's needed (seeing, running, digesting, mating) get replicated. Those that don't, do not.

Sexual differences

All the more wondrous, then, to discover what natural selection has achieved with human nature. The Darwinian exploration is still a fledgling science. But already it's yielding answers that we didn't even know had questions. What's the winning figure for ratio of waist to hips? Whose sweat has the smell of success? Who declares that baby looks like its dad? Why are mother and foetus locked in irresolvable conflict?

New though our science is, we're already pretty confident about some things. Fortunately, one of them is sex. Consider an oh-so-familiar sex difference that emerged in a study of American college students. Asked by a stranger for a date, 50 percent of both women and men agreed. But if they were asked, "Will you have sex with me tonight?" not one woman agreed. For men, this shot to 75 percent. When students were asked, "How long would you have to know someone before having sex?" the questionnaire had to be rescaled for males requiring only minutes or seconds. Not only are men willing to have sex with a perfect stranger; they're also willing with a very imperfect one. Another American study found that, for brief encounters, men (but not women) were willing to drop their standards as low as their trousers. They were ready to dispense with intelligence, humour, charm, honesty and emotional stability, and were willing to tolerate pretty well everything from unpopularity to bisexuality.

Why this difference between men and women? When natural selection shaped male-female differences, it didn't stop at muscles and naughty bits. It also shaped differences in our psychologies. Evolution made men's and women's minds as unalike as it made our bodies. Why? Think of it this way. Give a man 50 wives and he could have children galore. But a woman with 50 husbands? Huh! Generation after generation, down evolutionary time, natural selection favoured the men that strove most mightily for mates. The most successful were the most competitive, risk-taking and opportunistic. We are all the descendants of those winners.

Meanwhile, females faced nine months of hard labour, breast-feeding and rearing. A woman had to be far more picky about whose genes ended up partnering hers. Faced with the prospect of highly dependent offspring, she'd be on the lookout for someone who was not only fit and healthy but also had access to resources, resources he was willing to share. Nowadays a Rolex or designer sneakers, a diamond or a dinner, provide cues. But for our hunter-gatherer ancestors, who roved the Pleistocene plains with no excess baggage, what mattered were social resources. The resources of status, standing, reputation and respect. As Onassis noted, amassing a fortune is no end in itself: "If women didn't exist, all the money in the world would have no meaning."

The genes that built brains, structurally and biochemically, with tools for making these shrewd decisions, were the ones that got themselves replicated. Of course, natural selection doesn't download its strategic plans straight into our consciousness. Its instruments are emotions, priorities and desires. Behind each of these everyday human feelings are the calculations of natural selection, millions of careful years in the making.

For example, men, without knowing it, tend to prefer women with a waist-to-hip ratio of 0.7. This is a waist that is 70 percent of the hips (for instance, 24 inches to 36 inches). The total amount of fat that men like on their women varies from culture to culture. But, in all cultures, men like that fat distributed according to the 0.7 rule. Twiggy's skeletal form and Rubens's hefty muses share a 0.7 waist-to-hip ratio. So do dumpy Palaeolithic "Venuses," figurines shaped by our ancestors 28,000 years ago. Why? It's an ingenious fertility-detection mechanism. Waists and hips are shaped by sex hormones, oestrogen in particular. And the optimal hormonal mix for fertility is also the one that will sculpt that desired ratio.

While he's unconsciously applying his mental tape measure, she's unknowingly sniffing him out. An experiment with unwashed T-shirts has revealed that women prefer the smells of men whose immune systems are least like their own; the greater the difference, the keener she is. It's natural selection's way of endowing her offspring with a diverse immune system. This arms them against the sea of parasites and other biological nasties that have been the deadliest force throughout our evolution.

Both sexes have a predilection for symmetry. A body with matching right and left sides is an honest signal (because it is hard to fake) that the genes that built it are robust against those same invading pathogens. Intriguingly, women have higher rates of orgasm with physically symmetrical men. As for women's breasts, the larger they are, the more symmetrical they're likely to be. Why? Because breast-building requires oestrogen, and the larger the breasts, the more the developing body must have been awash with it. But oestrogen also suppresses immunity, making the woman vulnerable to pathogens. So breasts that are large and yet manage to be symmetrical signal that their owner's immune system is reliably robust.

Facial symmetry, too, is highly attractive. Beauty, far from being skin deep, is a Stone Age body scan, brimming with information about health and fertility. So it's no surprise, then, that there's unanimous cross-cultural agreement on what constitutes a beautiful face; even 2-month-old babies concur.

A chronic problem faced by our male ancestors was the uncertainty of paternity, whereas a chronic problem for the female was being abandoned by the baby's father. Sex differences in sexual jealousy reflect solutions to these different problems. Imagine your partner either having passionate sex with someone else or becoming deeply emotionally involved with them. Men get more disturbed about the sex, women about the emotions.

Sex differences in attitudes to virginity also reflect the difference in parental certainty. A wide cross-cultural study found that, universally, men value women's virginity more than women value men's. Cultural differences merely shift the extent to which people value virginity at all (a lot, for example, in Indonesia and Iran, very little in Finland and Sweden).

If we look at the problem of uncertainty of paternity, who remarks that the newborn baby looks like its dad? A study in Canada found that it was mothers and their relatives; would-be fathers suspended judgment; and nobody commented on resemblance to mums.

Sherlock Holmes read the agony column, "because it is always instructive." We can see that these types of instructions come straight from the Darwinian textbook. For instance, if we turn to lonely-hearts listings we find "Man seeks young, good-looking woman"; "Woman seeks older, financially stable man." If we study sexual fantasies we see that men think of anonymous multiple partners and have thoughts of bare skin, whereas women imagine someone familiar and tender emotions.

If we compare gay and lesbian relationships we can see that gay men look for novelty, variety, genital sex, pornography, youth and beauty. Lesbians want intimacy, stability and monogamy. Lets also consider adultery, or what we scientists politely call "extra-pair copulations" within "social monogamy." Males go for quantity, while females (who have established resources within "monogamy") go for quality, particularly high-quality genes. Across all its manifestations, human sexuality bears the stamp of evolved sex differences: preferences always diverge predictably.

Psychological differences

But it's not just sexuality. A funny thing happened on the way to divergent mating strategies. Natural selection created males and females so unalike that the differences don't stop at how fast you'll jump into bed. They pervade our psychology, shaping our interests, our values, our ambitions, our skills.

It's often said, for example, that men lack social skills. Don't believe it. It's just that their skills are, all too understandably, not what we call sociable. Men are masters at status-seeking, face-saving, assessing reputation, detecting slights, retaliating against insults and showing off. They are more persistent and competitive than females, more disposed to take risks, less willing to take no for an answer. Who overwhelmingly causes road accidents, climbs Everest, gets first to the moon, commits suicide? Who is the alcoholic, motorbike rider, scientist, stand-up comedian, child abuser, CEO, train spotter, pop star, gambler, smoker, bungee jumper, murderer and computer nerd? Men, of course.

Men take more risks even with fastening seat belts or smearing on sunscreen. Even as collectors of traditionally "women's" objects (such as dolls or kitchen gadgets), they do it biggest and best. Men outstrip women in deaths from smoking, homicide and accidents. Social scientists view these causes of death as "lifestyle" as opposed to "biology." But, in the light of evolutionary theory, speeding to death in a flashy car or seeing life out in a pub brawl is as much enmeshed in men's "biology" as is the higher male incidence of left-handedness, autism and colour blindness.

Put males and females in the same environment and their evolved psychologies can trigger hugely different responses. Boys thrive in competitive exams; girls could do without. Boys play competitive games, big on rules and winners; girls play cooperative games with consensual endings. Men buy records to complete the set, women to enjoy the music. Men punch through glass ceilings; women look for better things to do. Rich, successful men go for ever younger "trophy" wives; top women go for men even richer, more successful and older than themselves.

Darwin in the family


After the enlightenment of the agony column, Sherlock Holmes would turn dutifully to the criminal news. This, he felt, was not "instructive." Darwinian detectives do better. Consider the family. Criminologists are fond of remarking that it is the most dangerous place to be. If this were true, it would be a Darwinian scandal. Why? Remember that evolution is about genes getting themselves replicated. Sex is one way. Another is for genes to help copies of themselves in other bodies. But how can a gene "know" whether other people carry copies of it? One reliable way is to help kin. If I possess a "helping" gene, the chance that my sibling or my children will have it is 50 percent, my grandchildren 25 percent, my cousin 12.5 percent and so on. Some of our most cherished human values spring from this genetic reckoning. Whenever a mother braves hazards to save her child from drowning, or a brother donates a kidney for his sibling, so-called "kin selection" is at work. Hence the scandal if criminologists were right. We have evolved to lavish altruism on our kin, not to abuse or kill them.

But, of course, the criminologists are wrong. Take murder. For a start, most "family" victims are spouses. In genetic terms, not "family" at all, not targets of kin altruism. But what about infanticide? Children epitomise such targets: cargoes of their parents' genes, 50 percent each, sallying forth into future generations. Infanticide is therefore a profound challenge to evolutionists. So profound that it sent intrepid Darwinians trawling through the statistics in Britain and North America to find out what proportion of murdered children died at the hands of their genetic parents. The researchers discovered that stepchildren are about 100 times more likely to be killed than genetic children. What's more, genetic parents tend to use suffocation and end their desperate act with suicide, whereas stepparents use more brutal means and spare themselves. These differences also reflect the children's treatment when alive. What's suggested in "Cinderella" is shown to be true: having a stepparent puts a child at greater risk of severe maltreatment than any other known cause. Murder is a finale to deprivation, battering and abuse.

Nevertheless, from time to time headlines tell of a baby abandoned or of a teenage mother enduring a hidden pregnancy, giving birth alone and precipitating the neonate's death. Is disposing of one's own offspring always an un-Darwinian act? No. A human mother ties up so much of her potential reproduction in each child, not only before birth but after, that natural selection has equipped her with a psychology that prompts her to abandon newborns when conditions are unfavourable. It may be other hungry mouths, no helping hands, or a sickly physique. The loss of a newborn, however tragic, is less tragic than investing precious resources in an early grave and thereby depriving other children, already alive or yet to be conceived. Cultures find their own traditions for sanctioning this sad necessity: handicapped children are not suckled; twins, a double burden, are thought to be cursed; neonates are deemed human only after a lusty cry.

The killing of neonates alerts us to the fact that although deep common interests bind us altruistically to kin, this is itself a source of conflict. Ask any family lawyer executing a will: where there is sharing, there also lies competition. When your children insist indignantly that your decision is "not fair"; when they squabble over a toy that none really wants; when they behave "babyishly" after a sibling's birth; or when mother's keen to wean and baby resists, then bear in mind the following calculus of kin selection. A child's life is its sole bid for genetic immortality, and it values itself more than it values its brothers and sisters. Indeed, it is 100 percent related to itself but has only a 50 percent chance of sharing genes with sibs. For a parent, however, each child has the same value, a 50 percent relationship. The child will therefore always value itself, relative to its siblings, more than its parents do. The result, when it comes to parental investment, is inevitable conflict. A child is evolved to want more than parents are evolved to give.

Pregnancy puts its own peculiar twist on this conflict because, at this point in the child's life, even the parents' interests diverge. Indeed, the womb harbours strife between mother and father that would make a divorce court look peaceable. "He only wants me for my body!" women have cried down the ages. Darwinian analysis is now revealing that this is even more true after conception than before. Here's the evolutionary logic. Fifty percent of a foetus's genes come from its father, and our species cannot boast a history of reliable monogamy, so genes from that father might never borrow that womb again. Therefore, paternal genes in the foetus have evolved to exploit the mother's body more than is optimal for her. The father's weapon is the placenta, an invasive network of plumbing that wrests control of the mother's blood supply, enabling the foetus to grab more than its (maternally calculated) fair share of nutrients. This sets off a maternal-foetal arms race, with each side pouring ever more hormones and other weapons in to get only marginally more out. Occasionally, unable to marshal the normal evolved defences, a mother succumbs to one of the typical illnesses of pregnancy, such as diabetes. Only in the light of Darwinian analysis have we at last been able to understand these recurrent pathologies, otherwise so baffling in a "natural" process. They are glitches in an irresolvable conflict.

Now that we've got a baby handy, let's go beyond sex and the family to examine other bits of its mental toolbox. With every human birth, a baby Einstein, Brunel and Darwin enters the world. A newborn child is already well on its way to becoming an instinctive physicist, engineer, zoologist, botanist, linguist or psychologist. Tiny humans can anticipate the fall of a toy, distinguish speech from other sounds, recognise faces and interpret whether the face is happy, sad, fearful or angry. These are feats that decades of expensively funded research in artificial intelligence have notoriously failed to emulate. Why? Because babies are precociously primed by natural selection. Their brains are packed with information about the regularities of their ancestors' environment. This transforms what would otherwise be objectively unsolvable problems into the most humdrum of human activities.

Consider those little proto-naturalists. It was crucial for our foraging ancestors to track the lives of animals and plants; hence the potential hunter-gatherer in every child born today. Children only a few years old have an innate sense that living things obey distinct laws. They know that animals get around under their own steam and plants grow, whereas rocks do neither. They staunchly insist that a lion painted like a tiger is still a lion, a wolf in sheep's clothing still a wolf, whereas a coffeepot could, with sufficient tinkering, be turned into a bird-feeder because inanimate objects lack an intrinsic "essence."

As for the budding psychologist, we have seen that natural selection gave us an especially rich and intricate set of tools for dealing with those most enigmatic and protean bits of the living world: other human beings. There's an old joke about two behaviourist psychologists meeting in the street. "You're alright," says one to the other. "How am I?" It is thanks to our innate ability to mind-read that this is only a joke. Children universally possess an instinct for understanding intentions, attributing emotions, interpreting motives and, not least, predicting and manipulating behaviour. At one year of age, a baby monitors where others are looking and follows their gaze. By 14 months it is pointing and watching where they point. By four years, it can pass the following test: "Sally and Anne put a marble in a basket, Sally leaves the room, Anne moves the marble to a box and Sally returns to look for the marble. Where will she look for it?" The child will correctly reply, "The basket," having grasped that Sally will act on a belief that the child knows is false.

Evolution and the future

How does our ancient tool kit fare in the modern world? Darwinian science has yet to discover. For 99 percent of human existence we lived as hunter-gatherers. Then agriculture arrived and the future wasn't what it used to be. Our evolved bodies and minds were unchanged, but there were novel environments and cues they were not designed to cope with. How would they respond? For an inkling, go no further than your local fast-food joint. It is a monument to ancient tastes, to our evolved preferences for sugar, fat and salt. In our past, these were so scarce that we couldn't eat too much. Now our instincts are misled, resulting in the first epidemic of obesity that humans have ever known.

We are processing not only food but also information that we weren't designed to digest. We have initiated a huge inadvertent experiment on human nature. Think of our devices for choosing mates: exquisitely fine-tuned, but not, perhaps, to some of today's challenges. A recent study found that when people were shown pictures of beautiful and high-status people of the opposite sex, women became more dissatisfied with their own partner after seeing high-status males and men more dissatisfied with theirs after seeing beautiful females. We were evolved to assess beauty and status, and to calibrate our satisfaction against a few hundred people at most. Yet we are all now exposed daily to images of the world's most beautiful women or to enhanced images that are probably more beguiling than any woman any of our ancestors ever saw. Similarly, we are exposed to men of prodigious status. The world's richest and most powerful are helpfully contrasted with the most menial and downtrodden. These discrepancies are far greater than our minds were evolved to contemplate. Global communications amplify these invidious messages across the world.

Our species has been faced with unprecedented inequalities ever since agriculture enabled us to hoard resources. But in recent years the stakes have escalated. The game is increasingly "winner-takes-all." From the world's chess champion to the leading libel lawyer, a few places at the top command almost all the status, and, as rewards rise, the gap between top and bottom grows. Caught in this game are males who are evolved to value status, to be vigilant to cues, to strive to stay ahead. What impact might these novel inequalities be making on them? Might it be significant that, according to recent research throughout the developed world, countries with the greatest inequalities in income are marked by poorest health and earliest death? Is it relevant that illness and mortality are affected more by relative than by absolute living standards?

What of the future? It is often claimed that human evolution has ended because technology cushions us from disease and death. Equally often it is claimed that human evolution is accelerating because technology favours balloon brains on puny bodies. Neither is true. Natural selection's pace is slow; genes are plodding on with building bodies and minds in much the same way as they have for a million or so years, and that's how they'll continue for a long time to come. The adaptations that we bear tell us about long-lost worlds in which our ancestors dwelt. But those same adaptations can tell us about our future. For it is not to human nature that we should look for change but to the intriguing new responses, the innovative behaviour, that changing environments will elicit from that nature. This enduring thread of humanity reminds us that, however novel our environments, their most salient feature (for us and our descendants, as for our ancestors) is other human beings like ourselves, a meeting of evolved minds.

A version of this story first appeared in Time magazine. Copyright Helena Cronin.