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* Emotions are not primarily about feelings. Basically they are unconscious survival mechanisms - the conscious recognition of them is just a lately evolved sophistication designed to make life easier in a complex social set-up. Emotions are generated in the limbic system - the ancient, unconscious brain buried beneath the cortex. Incoming information is "tasted" here before it reaches consciousness and anything untoward triggers an appropriately protective physical reaction such as fight, flight, appeasement or paralysis. Conscious emotion arises only when information about the changing body state reaches the conscious brain.

* The hypothalamus is a cluster of minute organs that together produce our appetites, urges and bodily drives. The organ that is responsible for assertive (rather than passive) sexual behaviour is on average 2.5 times larger in men than in women.

* The corpus callosum - the band of tissue that connects the brain's two hemispheres - is relatively larger in women than in men. So is the anterior commissure, a more primitive connection between the hemispheres that links their unconscious areas only. This may be why women seem to be more aware of their own and others' emotions than men - the emotionally sensitive right hemisphere is able to pass more information to the analytical, linguistically talented left side.

* Men lose their brain tissue earlier in the ageing process than women, and overall they lose more of it. Men are particularly prone to tissue loss in the frontal and temporal lobes where brain damage is known to cause irritability and other personality changes. Women tend to lose tissue in the hippocampus and parietal areas that are more concerned with memory and visio-spatial abilities. Some imaging studies show that women use both sides of their brain when they try to work out a complex mental task, while men use only the side that is most obviously suited to it. This pattern of activity suggests women take a broad view while men are more incisive.

* The brain is moulded by its environment from the moment of conception. Hormone "baths" in the womb masculinise boys' brains by encouraging growth in certain brain areas and inhibiting it in others. The right hemisphere areas concerned with spatial tasks are enhanced by male hormones while the left-brain areas that process language are held back. Psychologist Simon Baron-Cohen of Cambridge University has suggested that this pre-natal skewing produces in all men a very mild form of autism. RC