"Testosterone is the masculine chemical rumbling at the restless core of maleness. It contours the male body, propels the masculine temperament, and instigates a wide, defining range of male behaviors. Present in much lower levels in females—roughly a tenth—testosterone impacts everything from our muscle-to-fat ratios, to people's strength and energy thresholds. Testosterone is a survival fuel that compels the sex drive. Agitating for release, it especially sends males into fits of arousal. Libido, it seems, is a hormone.

Propelling the hunter-warrior to competitiveness and acts of dominance, testosterone was and is the source of the "take charge" male attitude, the penchant for aggression, the origin of prickly male anger and incautious acts. Testosterone is a tyrant, an itch that will not go away. It drives men to take on projects, to achieve and to succeed, kicking its heels even higher with triumph. Extensive surveys of men taking large doses of injected testosterone point to short-term but near-universal reports of an invigorated sense of confidence and assertiveness; a livelier, focused intellect; increased mental clarity; tenaciousness and positive thinking. The hormone generates boldness, a risk-taking bent that can tilt toward fearlessness, even acts of valor—and fearsome excess.

The adolescent male is another testimonial to the hormone's power. Formative young males absorb testosterone rushes two to three times that of a vigorous male adult and up to twenty times their female counterparts. Trial lawyers, those windup aggression dolls who battle at court, register higher testosterone levels than their nonlitigator associates. Studies report that blue collar workers have more than white collar workers. Blacks have more than Caucasians. Violent prisoners, male or female, have more than the soft cases. Military professionals register higher than new recruits.

The ebb and flow of this pungent hormone also plays a role in social cohesion. Winning sports teams and their fans experience a spike in testosterone levels, while the losers suffer a reduction. This has the effect of easing postgame tensions since the brazenness among the defeated is cut back. While the winners dance a jig in the end zone, the losers nurse their loss. And get ready for the rematch.

Married men, reined in by wife and responsibility, score lower on testosterone than bachelors, especially in the weeks after their wives give birth. This is nature's way of promoting fatherly nurturance, cutting down the wandering male eye, and boosting marital fidelity. Hormones also may play a part in holding couples together as they age. Men escape the tyranny of this demanding hormone as they move deep into their thirties. As testosterone fades in maturing men, their estrogen levels gain added emphasis. The edge comes off. This turns older men toward mentoring and nurturing, and makes grandfathers a softer touch than dads. Conversely, as estrogen levels fall in mature women, their testosterone reserves take on greater importance. They pick up a boldness that harmonizes with their husbands' offsetting hormonal drop. Men get softer as they get older, women get harder.

Yet women never get close to male levels of testosterone. Nor is it apparent that this is a bad thing. But this difference certainly helps explain why so many of the social, economic, and cultural imbalances between the sexes seem so resistant to change. It's not just about adopting new attitudes and beliefs. Males and females approach life with different biological, evolutionary, and hormonal legacies. Out on the savanna, female requirements for boldness and risk-taking were limited mostly to protecting the nest. The same can't be said for hunters and warriors.

Men needed to be confident, even bold, prepared at times to throw caution to the wind. They needed to focus sharply and be tenacious in pursuit of resources, a mate, and status. Testosterone, for all its can't-sit-still restlessness, continues to confer advantages upon men in charged, competitive environments such as athletics, the military, and the risk-oriented, firing-line slots of modern organizations.

Testosterone initiates the male journey and makes the world go round. The chemical of initiative, it can also drive male energy toward chaos. In that sense, it may be the most dangerous drug in the world. As modern mobility shrinks the planet, drawing the neighborhoods of our Global City ever closer, and technological advances extend the destructive scope of individuals, we need to watch out that unbridled testosterone doesn't send the world spinning out of control."