Rewind back to the nineteenth century. A huddled group of anatomists pronounce a scientific breakthrough: ''The female of the species has a smaller brain.'' Reactions to this are varied.

Scenario One: Cue a chorus of smug baritone voices echoing around the nation and nodding in unison - ''Told you so!'' It was only a matter of time before the medical fraternity came to their senses and realised that size is all, concludes the more hirsute half of the population. If women didn't have such a minute brain, they wouldn't watch puerile soaps, moan about their male partners' communication problems, and make utterly pathetic attempts at car maintenance. Stands to reason doesn't it, grin our nascent men in white coats. Women act daft 'cos their brain is in the feather-weight, not heavy-weight, league.

Scenario Two: The less hairy 52% stand defiant, arms folded, and equally smug. A choir of sopranos cry out in protestation that the male brain - whatever its size - must be lacking in vital areas. If the brain is a big muscle then the boys ain't flexing theirs in the social areas that matter. Why else, reason the XX chromosomes, would men moan about minor ailments in the manner of felled deer, assume the Pavlovian response of leering at every babe that passes, and refuse to acknowledge the fact that men also get lost when driving in unfamiliar territory? ''Geezers!'' they sigh, and shake their lighter heads in pity.

The human brain and its influence upon the sexes' respective behaviour is about to receive yet another probing. Not since male anatomists posited the idea that women were less intelligent because their brains weighed less - neglecting to correct for the strong influence of body weight on brain weight - has the grey, wobbling matter been under such scrutiny. The BBC's The Human Body last week dived into the mysterious mushroom consistency of the brain, while tomorrow on Channel 4 a new series, Why Men Can't Iron, reveals how fundamental sex differences in the brain affect the way men and women think, feel, and act. Finally, we have A Guided Tour of The Human Brain, courtesy of Susan Greenfield, Professor of Pharmacology at Oxford University, published on July 4. Our collective matter is throbbing in sympathy at such an onslaught.

What all three of these textual and visual broadcasts agree upon is the difference between a male and female brain. Brain sex, if you will. In light of this discovery, the nature over nurture argument is gaining credence as scientists, and not sociologists, unfold the mysteries of what is the most complex object in the known universe.

It also translates into one of the most vitriolic and incendiary debates staged in male-female relations. After three decades of The Female Eunuch and Sexual Politics drip, drip, dripping into our politically correct consciousness that men and women are equal, some experts are suggesting the ultimate blasphemy. We're not.

Taken to its most fatuous conclusion in the Channel 4 series, this translates as: men can't iron because their brains aren't geared towards such an act. Likewise, women cannot do household maintenance because their brains aren't predisposed towards it.

So men behave badly because they just can't help it? The lads at loaded magazine must be burbling joyously in their beer barrels, while the phalanx of Kate Milletts seethe with rage. Isn't this the type of troglodyte argument we thought had disappeared with the maxi-skirt?

Ernie Govier is a professor of psychology at the University of East London and an advisor to Channel 4's Why Men Don't Iron. Govier puts the ''heretical'' claims into perspective. ''For almost 30 years it has been very difficult to talk about the differences between people because the environmentalists have held sway,'' he says, referring to the camp who believe in the influence of upbringing and social environment on a person. ''If in the sixties you said that men and women were different, it was taken to understand that you really meant men were better than women. That is not true of course, but people have to recognise that despite being equal under the law, our brains are programmed in such a way that men and women are predisposed to develop and behave in different ways.''

Due to technology like PET scans and modern imaging techniques, physical evidence - still hotly contested - which distinguishes the brain's sexual characteristics is now available. Govier explains: ''The debate at present is based upon a bridge that straddles the two hemispheres of the brain, called the corpus callosum. This is the bit through which the two sides of the brain talk to each other. In women this bridge is much thicker.'' This translates, he says, as better communication and language skills, apparent even in female babies who, studies have shown, have a wider range of noises than male peers. Unsurprising, when you consider that the part of the brain used for speech and language is about 25% bigger in females. He adds that new-born girls are far more sensitive to pain and touch and in later years, and their sense of smell is three times more acute than males. Where baby boys are

more taken by objects, the girls prefer people and faces.

He continues: ''We are also able to observe that the two sides of the female brain seem active, whereas in the male, the right side seems to do most of the work.'' This compartmentalisation in boys is also thought to aid them in the logic of mathematics. Whereas in females, says Govier, it explains why they excel at multi-tasking and why the average teenage girl's attention span is 15 minutes compared to a boy's five minutes.

Those in the scientific camp will tell you that these physical differences are thought to date back to prehistoric times when cavewoman honed the gatherer, home-building, language, and emotional skills. Caveman meanwhile concentrated on tracking his prey, and honed his physical aggression and skills with spatial relationships.

According both to Govier and the band of expert advisors deployed by Channel 4, these controversial claims are borne out by observational studies into male and female behavioural patterns. In Why Men Don't Iron, a series of experiments with a group of four to seven-year-olds shows the destructive and anarchic nature of boys compared to the co-operative and non-competitive nature of girls. Left to their own devices, the boys are incapable of sitting still for long, becoming territorial and running riot while the girls settle down and concentrate, solving problems by consensus. The main element in boys' games is competition. Indeed, the programme demonstrates the abysmal failure of a well-meaning family who give their children unisex toys only to find the boys beating each up outside while the girls play Barbie inside.

According to another of the programme's advisors, New York anthropologist Helen Fisher, this in-built competitive urge in males is due to a lack of ''brain brakes'', or seratonin levels. This means men are more impatient and impulsive. Combine these traits with the male brain's fuel - testosterone - and this is the outcome. ''Men's brains tend to have great acceleration and dodgy brakes,'' she reveals. ''This means they are driven to succeed, but often don't know when to stop.'' She concludes that the male brain is designed to get a bigger thrill out of risk and competition than the female. The dearth of female Formula One drivers, stuntwomen, and danger sportswomen seems to testify to Fisher's claims.

The task of holding the fort in defence of nurture over nature is left to the Professor of Behavioural Science at Glasgow University, Keith Miller.

''It is difficult to tease out exactly what is due to genetics and what is culturally shaped in our varying behaviour,'' he concedes. ''You do have to acknowledge that when the same opportunities and encouragement are given to women then they too will make in-roads into what was previously considered the male domain.''

He cites the marked rise in female doctors, girls' high-scoring academic records, and the medical trends in female businesswomen in the West. ''They are succumbing to stress and cardiovascular disease in the same way as their male counterparts,'' he explains. ''It would seem that women can be just as competitive as men and their health suffers in the same way.''

As the scientific and sociological factions square up for The Big Fight, does this really mean a doomsday scenario for male-female relationships? Ernie Govier is pragmatic. ''Men and women will never agree with each other as long as they keep believing in fairy stories that we are all the same,'' he concludes. Hopeless romantics, loaded readers, and marriage guidance organisation Relate therefore find themselves as unlikely bedfellows in the Great Sex Differences Divide.

n Why Men Don't Iron, Channel 4, starts tomorrow at 9pm.

GREY MATTERS

Chimpanzees have a brain equivalent in size to the engine of a Fiat 500, while a human brain is three times as large, more the size of a sports car engine.

n As a brain cell fires, the electrical impulse travels at 400 kph.

n The brain is composed of 100 billion brain cells or neurons.

n Together they can generate enough electricity to illuminate a light bulb.

n The brain uses up one fifth of all the food that we consume.

n The average person can remember a million different items.