We鈥檝e found real differences in the ways men and women think and behave 鈥� but which ones matter? 快猫短视频 puts things in perspective
AS IF bookstore shelves weren鈥檛 groaning enough under the weight of books revealing the differences between men and women, Donald Pfaff has recently added another to the pile. Ask why he felt compelled to throw yet another volume into this vast library and he will tell you that he was fed up with reading popular but 鈥渟cience-lite鈥� accounts. Authors who pretend that we are all alike annoy him as much as those who claim that men and women inhabit separate planets. Both are guilty of over-simplification, he fumes, and of treating the brain 鈥渓ike a lump of jello鈥� lacking the diversity we need to run modern societies.
In November last year, Pfaff, a professor of neurobiology from Rockefeller University in New York, was a keynote speaker at a conference titled , held at the European Molecular Biology Laboratory in Heidelberg, Germany. This was the latest in the annual Science and Society conference series, which aims to bring topical, cutting-edge biology to public attention 鈥� and it clearly hit the mark. This is a hot area of research but Pfaff was not alone in expressing frustration at how the findings are sometimes portrayed. Several other speakers noted that while we are happy to accept that men and women differ physically, when it comes to behaviour or the way we think, sexual politics quickly muddy the picture.
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The conference also revealed that when you strip away the politics it is surprising how much we actually do know. There is certainly enough science to address such contentious issues as whether women are innately bad at mathematics (they are not) and whether cultural indoctrination alone can explain why boys and girls tend to play with different toys (it cannot). Yet confused onlookers are often left with the impression that, when it comes to sex differences, everything is still up for grabs. So what鈥檚 the inside story?
For a start, we have learned a great deal about the biology that underpins sex differences. For years, the accepted view was that all embryos start out the same 鈥� the default sex being female. Then during the first trimester, in individuals that have inherited a Y chromosome, a gene called sry, for sex-determining region Y, switches on the development of the testes. These start pumping out testosterone and by the time a baby boy is born, the 鈥渄efault鈥� female brain has become masculine.
We now know that鈥檚 not quite how it works. It turns out there are 鈥減ro-female鈥� as well as 鈥減ro-male鈥� genes, and that sexual differentiation is governed by a delicate balance between the two. In 2006, for example, Pietro Parma at the University of Pavia in Italy, and colleagues, reported that a gene called r-spondin1 promotes the development of the ovaries, and that without it individuals who are genetically female grow up physically and psychologically male, although they have ambiguous external genitalia and are sterile ().
Biologists have also revised their views on the role of sex hormones. Testosterone in men and oestrogen in women were always thought to account for most of the biological differences between the sexes. While that remains the mainstream view, it is now clear that the effects of hormones and genes can interact, with implications for the wiring of the brain and, ultimately, for behaviour. Moreover, the contribution of genes can in turn be modified by experience: a child鈥檚 early environment can induce chemical modifications of DNA 鈥� so-called epigenetic changes 鈥� that without altering the actual sequence of a gene changes whether it is active or quiescent in a particular tissue.
The identification of all these sex-determining factors and their complex interactions has an important corollary, which is that sex determination is not over by birth, as was once thought. Both nature and nurture play a role in shaping the differences between men and women, nowhere more so than in the brain, which is constantly remoulded throughout our lives. Many now believe that there are critical periods when the sex of a child鈥檚 brain 鈥� and everything that accompanies it, including such things as the individual鈥檚 attitudes to love or food 鈥� is particularly malleable. By the time we reach adulthood there are numerous differences in structure between the brains of men and women, as revealed by brain-imaging studies (see diagram). These could explain why males and females show such different vulnerabilities to mental illness and learning difficulties (see 鈥淲ired for trouble鈥�), but as yet neuroscientists know little about how the structural differences translate into behaviour.
Spot the difference
That said, over the years psychologists have developed a good picture of which human behaviours show sex differences. What has emerged is a hierarchy of traits (see table) within which, Pfaff noted at the Heidelberg conference, there is one rather obvious pattern: 鈥淭he further you go from reproductive behaviour, the less impressive the sex differences.鈥� So, not surprisingly, at the top of the table are gender identity and sexual orientation, which both have a direct bearing on an individual鈥檚 chances of reproducing. Put simply, the vast majority of people who think of themselves as male are men, while those who consider themselves female overwhelmingly tend to be women. Likewise, most people who prefer their sexual partners to be women are men (and vice versa).
Nobody is going to object to that. But things get more contentious further down the scale when we start considering traits such as empathy and assertiveness. One way to cut through this is by comparing the extent of psychological differences between men and women with an obvious physical one such as height. As well as putting the size of various behavioural differences into perspective, this also gives a more dispassionate take on what the differences mean. Everyone would agree, based on their visual experience, that men are on average taller than women, yet there are enough tall women and short men in the world that height alone is not a reliable predictor of an individual鈥檚 sex. A similar rationale exists for behavioural differences.
Taking this approach, last year Jay Giedd and Judith Rapoport of the US National Institute of Mental Health in Bethesda, Maryland, pointed out that most of the effects of sex on behaviour are only around half the size of those on height (). Nevertheless, there are real and measurable differences. Play behaviour, for example, while varying far less between the sexes than gender identity or sexual orientation, is next in the league and differs by about the same amount as height. In practice this means that boys are on average more likely than girls to engage in rough-and-tumble play, or to choose a truck over a doll, but there are enough exceptions to that rule that it is not possible to predict a child鈥檚 sex from his or her play preferences alone (see 鈥淭oy story鈥�).
The areas where differences between men and women are about half that of height include aggression, empathy, assertiveness and cognitive skills such as the ability to mentally rotate an object. Further down the list come verbal fluency and mathematical attainment, which show far less variation between the sexes than we are often led to believe. And at the bottom of the chart are a bunch of traits commonly thought to be biased by sex but which in practice show no discernible difference between men and women. These include computational skills, overall verbal ability and leadership potential.
In other words, the picture science paints is one where sex differences are real but not deterministic. In certain areas men may tend to be one way and women another, but the role played by nurture and the environment in shaping these differences means that we may have more influence over them than we thought. That is certainly good news for anyone who believes that the workplace should be more conducive to women, and the classroom more boy-friendly. But there are no simple fixes, warns child psychologist and writer Susan Pinker, based in Montreal, Canada, who also spoke at the Heidelberg conference. There may nevertheless be things we can do 鈥� and a readiness to leave the politics aside and address the issues empirically is a good starting point. It is no longer good enough to say that boys will be boys, and girls will be girls. 鈥淭hat,鈥� says Pinker, 鈥渋s simply the path of least resistance.鈥�
Toy story
Why do girls prefer dolls and boys cars? Some put it down to cultural influences that prepare children to take on stereotypical gender roles as adults. Now consider this: male vervet monkeys prefer cars even though they have never been primed to do so (), and girls who have a hormonal disorder that means they produce too much testosterone prefer them, too. This suggests an innate component to toy choice, which may be amplified by socialisation processes after birth.
Intriguing new research by Margaret McCarthy at the University of Maryland in College Park points to the neurobiology underlying sex-specific play preferences 鈥� in rats, at least. Her group found that the amygdalae, twin brain structures that are important for processing emotional and social cues, contain between 30 and 50 per cent more of a type of brain cell called glial cells in female rats than in males. Male brains, meanwhile, had higher levels of endocannabinoids 鈥� naturally occurring molecules that stimulate the same neural circuits as the active ingredient in cannabis.
However, when the researchers injected day-old female rats with a dose of a cannabis-like substance, they found that after three days the proportion of glial cells in their amygdalae was the same level as in males. These females now played like male pups too 鈥� they played 30 to 40 per cent more than regular females, and indulged in much more rough-and-tumble play ().
The main structural differences between male and female rat brains all have parallels in humans, and researchers believe that all mammals have the same neural mechanisms underlying key survival behaviours. Yet whether such similar neurobiology underpins play differences among children is unknown.
What is clear is that human behaviour is highly adaptable, and this includes play and other behaviours that differ between the sexes. What goes on before birth and shortly afterwards is not the ultimate determinant, says McCarthy. 鈥淚n the human brain, we think there鈥檚 a lot that experience can do.鈥�
Wired for trouble
There are clear differences in the types of mental illness and learning difficulties that males and females experience. Boys are much more vulnerable to developmental difficulties than girls. For example, boys are between six and 10 times more likely to be diagnosed with Asperger鈥檚 syndrome, four times as likely to be affected by language disorders such as dyslexia, and a conservative estimate suggests that boys are twice as likely to suffer from attention-deficit hyperactivity disorder.
The picture is more chequered for adults, but the differences are still dramatic. Major depression is twice as common in women, while men are more susceptible to alcohol dependence and antisocial personality disorder. Even in conditions for which the prevalence is the same in both sexes, such as bipolar disorder and schizophrenia, there are differences in age of onset and symptoms.
Melissa Hines, who studies gender development at the University of Cambridge, reckons sex differences in such conditions are the result of different vulnerabilities due to the distinct ways in which those brains are wired. We know, for example, that the amygdalae, a pair of brain structures important for processing emotions such as fear and aggression, are bigger in men, while the hippocampi, critical for memory, are bigger in women. Such brain differences are shaped by a combination of genes, hormones and the environment. 鈥淚t鈥檚 all of these things together that make the final outcome,鈥� says Hines.
The origins of antagonism
The evolutionary roots of sex differences can often be found in sexual selection 鈥� the proliferation of traits, such as the peacock鈥檚 tail, that are considered most attractive by members of the opposite sex. Behavioural differences among humans are no exception.
Take competitiveness. Male-male competition is a feature of 90 per cent of traditional human societies, but the Jivaro people of the Amazon take the prize for bellicosity, with a breathtaking 60 per cent male mortality rate due to ambushes, raids and other warlike behaviour. However, for the Jivaro man who wins the battle, the rewards are high: his social status increases, as does his desirability to women, making him more likely to pass on his genes. The same competition exists in industrialised societies, says David Geary at the University of Missouri in Columbia, although it is often disguised as a contest for wealth. 鈥淭he ambition, the aggression, the wanting to outdo the other guy is all the same, but the way of expressing it is different,鈥� he says.
Of course sexual selection has also shaped women, and the resulting differences between the sexes are bound to lead to antagonism. 鈥淚f you鈥檝e got the two genders being selected on different traits, and they have to come together to produce offspring鈥� clearly there鈥檚 going to be conflict,鈥� says Monique Borgerhoff Mulder at the University of California, Davis. The simple fact that a woman must nurture her fetus for nine months whereas a man鈥檚 input to procreation may be far smaller can explain differences in priorities and attitudes in key areas such as parental investment and number of sexual partners.
Nevertheless, biology is not destiny 鈥� some women are more sexually promiscuous than some men, for example. What鈥檚 more, says Borgerhoff Mulder, social environments exist where women who have several sexual partners are more reproductively successful than those who are monogamous. Likewise, Geary points out that in societies in which women earn their own livings, male wealth is a less potent status symbol and men compete for it less. Evolution may explain different aptitudes and behaviours of men and women, but it does not determine them.
- Man and Woman: An Inside Story by Donald Pfaff (Oxford University Press, 2010)
