Is it nature or nurture that makes men and women behave differently? New research into how our brains work highlights just how complicated this issue really is. Aisling Irwin reports.

Men, can you tell from a glance at a woman's face whether she is happy or sad? Apparently not. Show a woman a photo of a happy and a sad man and she can tell you what they are feeling. Show a man a photo of a happy woman and he does fine - but show him a sad woman and he will misinterpret her emotional state.

There is no getting round it: studies show that men are different from women, and not just in terms of their physical endowments. Give a man a three-dimensional puzzle and he will be more dextrous with it than a woman will. But he will fare worse on certain verbal tests. Men do best, and worst, at maths. And the difference in aggression between the sexes dwarfs differences in height or weight.

Scientists and social scientists have been trying to answer the potent question about these and other differences: what causes them? So far they have produced ammunition for those who prefer to believe sexual differences are innate - but also ammunition for those who think we are conditioned by our environment to behave in certain ways.

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Most academics agree that the nature/nurture debate was won decades ago. It is obvious both are at work in influencing behaviour. But the exact nature of the relationship is proving hard to untangle. Recent attention has focused on the structure of the brain. The moderates among the brain experts will say little about sexual differences with utter conviction. But it is obvious that male brains are about a third bigger than females' simply because the male body is a third bigger.

Inside the brain, no one thought there might be significant sex differences until the mid-1970s when Arthur Arnold, now professor in the department of physiological science at the University of California in Los Angeles, pried into the brains of canaries and zebra finches to find the regions that control their melodious songs. He discovered that males had much larger control areas than females, corresponding to the fact that males have a larger and more complicated repertoire of songs. Fifteen years later, after a deluge of sexual differences discovered in the brains of fish, frogs, lizards, rats and other vertebrates, scientists started to claim that they had discovered differences in human brains.

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Yet this hypothesis remains controversial. Studies are contradictory, says Roger Gorski, professor of neurobiology at UCLA. They have been clouded by ignorance of distorting factors: male and female brains age differently; the brains of right-handed and left-handed people differ; brains are altered by the effects of disease and, ultimately, by death. There are differences, but they are much smaller than those found in rats. Gorski warns: "There's no human brain-region you can look at today and say 'that is a male or a female brain' - there's too much overlap."

Animal studies show that most brain differences are in place before a baby is born. Brains are programmed to develop into female brains unless male hormones start circulating, steering the brain towards maleness. But shape and size are not the only factors distinguishing male and female brains. There is also brain activity. Ruben Gur, professor of psychology in psychiatry at the University of Pennsylvania, has been using new imaging techniques to watch live brains as they cogitate. He has found that men's and women's brains age differently. As men age, their brains shrivel three times faster than women's do. And their verbal memories, visuo-spatial skills and ability to pay attention also erode. Women of the same age keep these faculties.

When Gur scanned the brains of men and women between the ages of 18 and 45, he discovered that, as they get older, women slow down the rate at which they burn energy in the brain. The slowdown in men is much less pronounced. Gur thinks that men overwork their brains. The chemical byproducts that this produces kill surrounding cells so men's brains deteriorate faster.

Gur says that the smooth decline from 18 to 45 is too clear a line to have an environmental cause. "The kind of differences that we see suggest neurobiological programming operating rather than the effects of the environment," he says.

Contrast these discoveries about innate brain differences with work by Jacquelynne Eccles, a social scientist at the University of Michigan. She is investigating why girls do not go into science or engineering. She has studied schoolchildren and their parents, building on research that seeks to predict how well children in early elementary school will perform in late elementary school. Previous researchers had discovered one predictor that is more powerful than how well the child is doing in its early years: the expectations of the child's parents. So Eccles decided it was crucial to investigate what moulds those parents' expectations.

"Clearly, parents' perceptions in the academic domain are related to objective information provided by the school about how well their child is doing," she says. "But we are interested in identifying the other more subjective influences."

When she looked at maths, she found that gender differences in achievement are small and do not emerge before secondary school. Nevertheless, elementary school children's parents believe their sons are better than their daughters at maths. In English and sport, girls do differ from boys in achievement, so Eccles controlled for these differences. She still found parents expected daughters to do better at English and sons to do better at sport.

Eccles divided thousands of parents into three groups: those who think boys in general are better at sports and maths; those who think girls are better; and those with no opinion. She found that if a daughter of parents in the first group did well in a maths test, the parents would distort the reasons why, saying she had worked hard or she had a good teacher. If a son did well, however, parents said the boy had talent. This applied even though boys and girls put in the same amount of work. There were similar findings for sport and reading.

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Parents' expectations of their children are obviously influential, but not just because of what they say to them. And here we witness the biological and the social interacting. For, if you are a boy and a little better at sport than your sister, and your mother has distorted her views of your ability so that she thinks you are very good at sport and your sister is very bad, and she sends you out to play football while your sister is encouraged to do something else, then you will become good at football and sport in general, while your sister will not.

None the less, Eccles says that she has no quarrel with the discoveries of scientists like Gorski. "Human beings have been selected to be adaptive," she says. Of Gur's imager, in which men's brains overwork and women's do not, Eccles says:"Men may be trained to concentrate on one thing, women trained to switch attention quickly."Gur, who does believe in a nature/nurture mix, says this is possible, but unlikely.

Arnold says that the fact that there are biological differences in the brain that are created by an early hormonal environment does not undermine the "equally strong idea" that there are environmental influences on the sexual differentiation of human behaviour. He adds:"Ibelieve that the two forces are additive rather than interactive and they have both been working in the same direction."

But there is intriguing work that is beginning to show that we may never be able to break down sexual differences into a simple equation of x parts nature, y parts nurture. The two may interact and change each other in complex ways. The mother rat, for example, treats her daughters differently from her sons almost immediately they are born. Presumably she is innately programmed to do so. But the result is that, even at three weeks of age, the female rat has experienced a different environment from the male. Nature causes nurture. And perhaps nurture, in turn, could cause biological changes that would later be regarded as "innate".

If rats are put into a stimulating environment, their brains get heavier. An Illinois scientist, Janice Juraska, has looked at what happens to brain cells, called myelinated axons, in one region of the brain's corpus callosum. If the rats have spent time in an enriching environment the number of axons increases. But the numbers increase more in the female than in the male. Does this mean that females have more potential to be improved by stimulation? When rats were put into an impoverished environment the number of myelinated axons declined - but only in females. In males they stayed the same.

Melissa Hines, reader in psychology at Goldsmiths College, London University, says these findings could show that female rats are more socially malleable than males. The implications for human females are mind-boggling. But the rest of Juraska's research throws up a different emphasis. When Juraska conducted the same experiments but studied changes in other areas of the brain she found the same effects - but this time the number of axons increased in the male rats rather than the female.

Hines thinks we are going to find that each behaviour difference, each brain difference, has its own aetiology with its own proportion of nature and nurture and its own type of interaction between the two.

In an attempt to discover the relationship between the effect hormones have on our brains before we are born, and our behaviour as children, Hines studied children with genetic disorders linked to abnormal levels of hormones before birth. She compared girls exposed to increased levels of male hormones before birth with normal girls. She found the genetically afflicted girls were more likely to behave like boys in two ways: they played with boys' toys and chose boys as play partners. But Hines was also watching for whether they indulged in "rough and tumble"play, as boys and young male animals do - they did not.

Hines says that it is possible that the first two behaviours are "more" innate than the third. The children's parents might, therefore, intervene to damp down rough and tumble play, but not so successfully as to prevent the other two behaviours.

Whatever your ideological preference, whether for explanations leaning towards nature or nurture, it can be supported by the work of these scientists.

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But consider another study by Eccles, tackling innate male ability to excel at maths. She held a maths test and found men did better than women. Then she repeated the test, but telling women it had been specially devised to avoid all topics in which men do better. When the results came out, the women had done just as well as the men.