They say men are from Mars and women from Venus. Now, a group of researchers has backed this adage by providing scientific proof that the brains of the two sexes are physically distinct.
The study suggests that men and women show differences in behaviour because their brains are physically distinct organs.
Male and female brains appear to be constructed from markedly different genetic blueprints, the team says.
According to a review of recent neurological research appearing in this week's New Scientist magazine, it is becoming clear that the brains of men and women show numerous anatomical differences.
Some of these divergences 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.
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," the Independent quoted Dr Larry Cahill of the Centre for the Neurobiology of Learning and Memory, at the University of California, Irvine, as saying.
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.