KEY POINTS:
Men and women show differences in behaviour because their brains are physically distinct organs, new research suggests.
Male and female brains appear to be constructed from markedly different genetic blueprints.
The differences in the circuitry that wires them up and the chemicals that transmit messages inside them are
so great as to point to the conclusion that there is not just one kind of human brain, but two, according to recent neurological studies.
Men may be from Mars and women may be from Venus, and since the American psychotherapist John Gray wrote his famous book, in 1922, on the idea, it has been a commonplace to think of men and women as being from different planets in terms of their emotional responses.
But until recently, these differences were often explained by the action of adult sex hormones, or by social pressures that encouraged males and females to behave in a certain way.
Increasingly, however, these assumptions are being challenged, according to a review of recent neurological research appearing in this week's New Scientist magazine, and it is becoming clear that the brains of men and women show numerous anatomical differences.
Some of these divergences, the review by Hannah Hoag suggests, could explain a number of mysteries, such as why men and women are prone to different mental health problems, why some drugs work well for one sex but have little effect on the other, and why chronic pain tends to affect women more than men.
Although it has long been known that there were some male-female differences, it was thought they were confined to the hypothalamus, the brain region involved in regulating food intake, fighting and the sex drive, among other things.
But it is becoming clear that the relative sizes of many of the structures inside female brains are different from those of males.
One study, by scientists at Harvard Medical School, found that parts of the frontal lobe, which houses decision-making and problem-solving functions, were proportionally larger in women, as was the limbic cortex, which regulates emotions.
Other studies have found that the hippocampus, involved in short-term memory and spatial navigation, is also proportionally larger in women than in men n "perhaps surprisingly, given women's reputation as bad map readers" says the New Scientist review.
Proportionally larger brain areas in men include the parietal cortex, which processes signals from the sensory organs and is involved in space perception, and the amygdala, which controls emotions and social and sexual behaviour.
"The mere fact that a structure is different in size suggests a difference in functional organisation," says Dr Larry Cahill of the Centre for the Neurobiology of Learning and Memory, at the University of California, Irvine.
One area of research concerns the brain's pain-suppressing mechanisms, and points to the fact that they may be organised differently in men and women.
This would explain why women can suffer long-term pain more, and why there can be sex differences in response to opium-derived painkilling drugs.
The study notes: "Women get more relief from the opioid painkiller nalbuphine compared to men, whereas in men morphine is more effective and nalbuphine actually increases the pain intensity." It is possible these findings could lead to new painkillers being developed that are tailored to be more effective in women - but that is some way off.
Mental health is another area where real brain differences may offer explanations.
Women are diagnosed with depression twice as often as men, and this may be linked to relative levels of the neurotransmitter serotonin.
Boys, on the other hand, are more likely than girls to be diagnosed with autism, Tourette's syndrome, dyslexia, attention-deficit disorder and early-onset schizophrenia.
The review reports that Margaret McCarthy of the University of Maryland in Baltimore believes that hormone-like substances called prostaglandins, which help masculinise the male brain around the time of birth, may be partly to blame.
Drug abuse is a third area where brain differences may explain differences in behaviour.
Men are almost twice as likely as women to use cocaine, for example, (possibly due to social factors).
When women take it they get addicted more quickly and have a more severe habit when they seek treatment.
One of the reasons why physiological differences between male and female brains have not been widely noted before may be that most of what we know about the brain comes from studies of males, animals and human volunteers.
"If even a small proportion of what has been inferred from these studies does not apply to females, it means a huge body of research has been built on shaky foundations," the review comments.
Professor Jeff Mogil from McGill University, in Montreal, Canada, who has demonstrated major differences in pain processing in males and females, puts it even more forcefully.
He is astonished that so many researchers have failed to include female animals in their studies.
"It's scandalous," he said.
"Women are the most common pain sufferers, and yet our model for basic pain research is the male rat."
- THE INDEPENDENT