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 (Evolution and Human Behavior, vol 23, p467), 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 - 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 5O 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 - 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 - they played 30 to 40 per cent more than regular females, and indulged in much more rough-and-tumble play (Proceedings of the National Academy of Sciences, vol 107, p 20535). 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.
For years, the accepted view was that all embryos start out the same - the default sex being female. Then during the first trimester, in individuals that have inherited a Y chromosone, 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 "default" female brain has become masculine.
We now know that's not quite how it works. As it turns out there are "pro-female" as well as "pro-male" 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 (Nature Genetics, Vol 38, p 1304).
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's 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 alchohol 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. "It's all of these things together that make the final outcome." says Hines.
As for Sun Ra , he doesn't have very much to do with this post at all. But the Mars/Venus thing always makes me think of him. And he did come from Saturn. Here's my favourite tune by him, anyway: