In our series on the Big Science Questions, Aisling Irwin looks at the evidence for differences between the sexes beyond the basic mechanics of reproduction.
"Why can't a woman be more like a man?" Henry Higgins demanded in the musical My Fair Lady, to the instinctive sympathy of his fellows. After all, men are so much better at the important things in life. They know the virtue of silence. They run with grace, throw a ball with accuracy, park a vehicle with perfection at first try. They don't cry or take offence over trivial issues.
Over the past decade scientists have been adding to the list of male talents but also compiling a formidable index of female accomplishments. Women, as Higgins's protege Eliza Doolittle amply demonstrated, are the more articulate sex - fluent and expressive, they are mistresses of grammar and superior at reading. They are more subtle creatures, capable of judging the moods and intentions of others with finesse and running a web of complex social relationships.
Humorists and philosophers have written of the sex difference for centuries, most believing that they described the immutable. It is only this century that the belief that society moulds its own stereotypes has gained real power. Applied to gender, the idea took grip in the 1960s as social scientists revealed the degree to which women's behaviour is dictated by society, in particular role models and the media. Many social scientists discounted any role whatsoever for biology in the creation of gender differences - apart from differences in reproductive machinery. They now stand accused of dogmatism by a biology-based discipline that is trying to reclaim some territory.
Neo-Darwinists have to tread cautiously. Their predecessors - social Darwinists - used Darwin's theory of natural selection to provide an apparently scientific basis for slavery, racial discrimination and conquest. Today they call themselves evolutionary psychologists and their position on gender differences is this: men and women, driven by their distinct biologies to adopt different strategies for reproduction, have evolved different behaviours, values and ways of perceiving the world.
It all starts in the womb with a great rinsing of hormones that trigger, just 12 weeks after conception, the diverging of ways. In the uterus, the brain of the early foetus is innately female until this moment; then boy foetuses secrete male hormones known as androgens. As these hormones surge into the brain, they shape and reorganise it, emphasising some neural networks and suppressing others. Maleness has begun.
By withdrawing a little fluid from the womb, scientists can measure the exposure of an individual foetus to androgens. They can then try to correlate their behaviour as babies with their early hormonal environments.
Cambridge scientists Svetlana Lutchmaya and Simon Baron-Cohen have done just such a study. They selected a behaviour thought to be of major importance in normal social development: making eye contact with other humans. Evolutionary theory predicts that infant girls will engage in more eye contact than boys because of a deeper fascination with faces and emotions that is a prelude to developing greater social skills.
They found that at 12 months, babies who had experienced the weakest androgen environment in the womb were capable of the greatest amount of eye contact with their parents. These superior communicators were generally girls, but there was overlap between the sexes, reflecting overlap in the amount of androgen exposure in the womb.
Findings such as these may not directly indicate that girls are born with better social skills. It is possible that boys and girls are born with gender-dependent interests, motivations to seek out different experiences. Girls, more interested in observing expressions and emotions, gradually become more socially adept. This may be reflected in permanent brain changes.
Studies of three-year-olds find further differences. For some girls the rinsing in the womb goes awry and, as a result of the condition known as congenital adrenal hyperplasia, they are exposed to unusually high levels of androgens. Compared with unaffected girls, CAH girls are more competitive athletically, according to Melissa Hines, psychologist at City University, London. They prefer mechanical toys to dolls; rough and tumble play to play-caring for infants.
By the age of eight to 11, there is further evidence of a parting of ways and the development of spatial skills in males. Boys of that age roam more widely than girls. This is partly because they are given more freedom by their parents, but studies have tried to exclude this influence. These have found that, although girls are less keen to explore so widely, their navigation skills improve if they are encouraged to roam. Again, the different environmental experiences that boys instinctively seek may polish certain brain circuits and diminish others.
The hormonal effect continues into adulthood. Researchers have found that women's language abilities fluctuate with their oestrogen cycles, while men who change sex improve in some measures of language ability after hormone treatment.
The link between hormone levels and behaviour has been easier to demonstrate than that between brain structure and behaviour. Some scientists believe they have found structural differences between male and female brains, but Roger Gorski, neurobiologist at the University of California, Los Angeles, says the research is "very crude".
Scientists do seem to have found consistent differences in brain activity. Perhaps the most well-known example comes from the imaging of male and female brains while they do language tasks. Most women use both the right and left hemispheres of the brain, while most men use just one hemisphere. It is thought that the use of two hemispheres allows flexibility, fluency and the ability to perceive connections more easily.
For those who accept that the differing behaviour of girls and boys does have a biological component, the next step is to understand why. Evolutionary psychologists find an answer in sexual selection. This is the second great mechanism of Darwinian evolution, the first being survival of the fittest. It is not survival alone that guarantees a gene will be passed to the next generation - reproductive success is also essential. Thus, genes that enhance reproductive success are selected as the generations go by. Crucially, genes that confer reproductive success on women will differ from those that confer such success on men. This is because of the basic biological realities of reproduction. However hard she tries, a female can only produce about one child a year, while a man's reproductive possibilities are almost infinite. The two realities call for different mating strategies.
For most of the animal kingdom, including humans, methods of mating have two distinctive components: competing with members of your own sex for access to mates and/or choosing your mate from what is available. Evolutionary psychologists have thus settled on four principal mechanisms by which to understand how male-female differences arose: male-male competition and female choice (the most common pair), female-female competition and male choice. The degree to which each of these mechanisms dominates depends on the cultural setting.
In many species and, it is argued, human societies, competition for access to females is the dominant activity in a male's life. Success can make the difference between monopolising access to many females, on the one hand, and not producing a single son or daughter, on the other. Genes that help men achieve social dominance, for example, through superior fighting ability or political skills, will flood the next generation.
Females, traditionally vulnerable during pregnancy and early motherhood, and investing heavily in the wellbeing of a smaller number of offspring, have more success if they select males who can provide them with material resources and social stability. Thus female discrimination brings certain male characteristics to the fore - in pre-industrial societies, success in hunting, for example, for which navigational abilities and athletic prowess are indispensable.
The third mechanism, female-female competition over desirable mates, is thought to be primarily linguistic, according to Liverpool University psychologist Robin Dunbar. The conversations of girls and women often exhibit the tendency to try to exclude females who are perceived as rivals - the roots, perhaps, of gossip and "bitchiness". Gossip about sexual infidelity can spoil a woman's marriage prospects.
One of the more sensational discoveries of recent years relates to the final force - male choice. One study has shown that men all over the world prefer women to have an hourglass shape. More precisely, they like a waist-to-hip ratio of 0.7. Evolutionary psychologists argue that this ratio reflects a combination of health, youth and fecundity. A ratio of above 0.85, for example, puts women at risk for a number of physiological disorders and lessens their chance of conceiving.
It is important to understand that most evolutionary psychologists claim that this is only the beginning of understanding gender differences. Biologically based tendencies can be tuned upwards or downwards, redirected, counteracted or suppressed by environment. Culture can sculpt the instinct for male competition into homicidal violence or the pursuit of MBAs. We differ from the opposite sex because evolution demands it: hormones modify the raw material. What we make of that material depends on society.