Testosterone has long been held responsible for macho aggression. Now it's being touted as the great male behaviour manipulator. But has it been miscast? Deborah Holder on a few hormonal myths (including that one about bald men)
IT'S SAID to separate the men from the boys and the boys from the girls. Paula Yates boasts that Michael Hutchence is propelled by it. Teenage boys tell their sweethearts they're driven wild by it. Bald men have more of it. Most recently, Don EIium, disciple of the American Men's Movement and author of a US bestseller, Raising Sons, described testosterone as "the greatest manipulator of behaviour the world has ever known", challenging the traditional notion that women are the sex at the emotional beck and call of their hormones.

Research into testosterone, however, although it confirms some of the myths, is disproving others. Testosterone is supposedly responsible for sex drive and aggression. T-levels are seen as a measure of machismo - the reason men drive too fast, have to have sex and enjoy a good scrap after the footy. Testosterone often gets a bad press, implicated in everything from sex crimes to little boys pushing girls off the climbing frame.

In fact testosterone has three main functions. As the key male hormone, it is first produced in the womb by the foetal testes during the first three months of pregnancy. By its presence or absence, testosterone determines whether the embryo will develop into a boy or girl. No testosterone and it's a girl, almost by default.

The second function takes place during the baby's first six months. "It acts as a kind of insurance policy," says Fred Wu, a senior lecturer at Manchester University and leading researcher in the field. "If the testes are not properly descended, testosterone encourages them to do so." Testosterone also "masculinises the brain structures, although we don't know to what extent these anatomical changes affect behaviour."

Testosterone then goes to ground, reappearing over a decade later, as boys approach puberty. Levels of testosterone increase over four to five years, peaking in the early twenties and spiralling down from then on. Testosterone brings about a growth spurt, the development of such secondary sexual characteristics as body-hair growth and muscle, and a greatly increased interest in girls (and not for their keen brains or sparkling personalities).

Research shows that males at their peak, or those with high T-levels, experience what psychologists call, "more spontaneous sexual thoughts" - "it pops into their minds at the bus stop, in lectures, indeed whenever a female is nearby and sometimes when she isn't," says John Archer, a psychologist at the University of Lancashire. "But what they do with those heightened sexual urges is another thing." No way can high T be held responsible for sexual harassment or worse, explains Professor Archer, as social conditioning, reason, and all our other faculties are perfectly capable of overriding a sexual urge.

While experts agree that sex drive is squarely down to testosterone, they are less convinced of a link with aggression. "For a start it doesn't kick in until about 13," says Archer, "so what's happening earlier is unlikely to be linked to testosterone."

He also points out that studies of personality have shown us to be depressingly consistent creatures; the person we see at seven or eight is fundamentally the same as the one we see at 20 and 40.

Research looking specifically at aggression in boys as they grow older has also failed to detect an increase after the onset of puberty. Aggressive kids may well go on to make aggressive adults, but it doesn't seem to have much to do with T-levels. "Testosterone can only act on what is already there," says Archer.

Men have widely differing T-levels. "The man with the highest T-levels will have three times as much as the man with the lowest. Whatever your T-level, it can be temporarily raised by certain activities like anticipating sex, having sex, a big win on the lottery or winning a fight, and vicarious winning experiences, like watching your team thrash the opposition."

Archer believes this may explain the few studies that do link aggression with testosterone. "Maybe researchers are seeing the equation the wrong way round," says Archer, "and raised T-levels are caused by a successful aggressive encounter rather than being the cause of one."

However, although testosterone is not thought to be linked directly with aggression, Archer says, it could be linked with such behaviour as impulsiveness, risk-taking, and the need for immediate gratification - all of which may express themselves in a pro-social way, such as mountain-climbing, or an anti-social way, like joyriding.

Anecdotal evidence confuses the issue even further. A red-blooded 28- year-old, Ed, says he can feel a clear link between testosterone and agression. "To be blunt, if I'm not getting as much sex as I would like I become moody and aggressive. When I do eventually have sex, I feel a huge sense of release and from then on I'm much pleasanter to be around."

Like Archer, Dr Wu of Manchester University defends testosterone against links with aggression, citing experiments in which synthetic testosterone was used to treble T-levels with no effect on aggression. Giving testosterone to men can have almost the opposite effect, says Wu. When men who cannot produce testosterone are given artificial doses, they become calmer rather than more aggressive, because they feel more at ease with themselves.

Wu feels more research is needed to dispel the myths and misconceptions that obscure our understanding of testosterone. The case of Stefan Kiszko of Rochdale is an extreme example. Kiszko spent half his life in jail for sex crimes he was later found not to have committed. The case was strongly inflenced by the knowledge that earlier in his life Kiszko had been given testosterone injections to correct a deficiency. People put two and two together and got five, completely overlooking the fact that his condition also meant he could not produce sperm, though sperm had been found by the victims.

Wu is now concentrating on developing new preparations made from plant- based synthetic testosterone, to help men with low testosterone levels, and also to help produce a male contraceptive.

And finally, it's time to clear up the biggie - do bald men have higher sex drive? "The link is actually stronger between body hair and sex drive," says Archer. "Bald men often have a lot of body hair and strong beard growth - these things are linked to testosterone and therefore sex drive." He admits that he himself is bald.