By Ann Woolner
April 24 (Bloomberg) -- Of the 41 people who have taken guns to U.S. schools and opened fire since 1996, 40 of them share one trait.
They were born with the Y chromosome.
Maleness is the only characteristic that is common to this group, with race -- Caucasian -- coming in second. They are of different educational levels and regions. They are low achievers and high achievers. Their motives and mental states run the gamut. The youngest was 6. Another was 53.
Together, those 40 men and boys killed 94 people on campuses, plus four more in the hours preceding their school shooting episodes. They have wounded scores more and traumatized thousands of others.
I'm not saying boys are born killers. Only a miniscule fraction of them grow up to open fire at school or anywhere else. And chemistry isn't necessarily destiny.
And yet, whether sparked by jealousy, retribution, psychosis, insecurity or hatred toward women, U.S. school shootings happen with regularity -- 36 in 11 years -- and all but one were committed by men or by boys.
Maleness matters in all kinds of killings. Eighty-eight percent of U.S. homicides from 1976 to 2004 were committed by men, according to the most recent Justice Department statistics. For serial killers, the percentage rises to 93 percent.
Whatever you have heard about the so-called feminization of the American male or about increasingly aggressive women, the lopsided nature of which sex kills the most remains.
``When it comes to the most serious form of aggression, murder, the gender gap is actually wider now than it was a few years ago,'' says James Fox, a Northeastern University criminal justice professor in Boston who has written about killers.
I know. Most men channel aggression into perfectly acceptable activities. They build companies, enforce laws, repair roads, play sports, advocate causes.
But women manage to do those things, too. Yet when violence erupts, chances are overwhelming the aggressor is a man.
``It's true about murder,'' Fox says. ``It's true about crime in general.''
So what is it about men, anyway?
A leading testosterone researcher, the late Georgia State University Professor James M. Dabbs, found that the higher the testosterone level, the more violent the person is likely to be.
Testing more than 4,000 veterans, for example, he found that the 10 percent with the most testosterone were the most probable troublemakers. These were the guys likely to have misbehaved as schoolchildren, break the law as adults, use drugs and alcohol, go AWOL from the Army and have 10 or more sex partners in a year, Dabbs found.
He tested male and female inmates in separate research projects and found that in both populations the most violent had the highest T-levels.
Child psychiatrist David Shaffer of Columbia University says it's not clear from testosterone research whether the hormone itself sparks aggression. It could be that, because those with more of it are larger and more muscular, they find more success at physical aggression and therefore engage in it more.
He says there are other biological reasons for the gender gap. Men are more likely than women to lack metabolized serotonin, which is the neurotransmitter that acts as a calming influence, says Shaffer, chief of Child and Adolescent Psychiatry at Columbia University Medical Center in New York.
Incarcerated marines showed a dearth of the stuff in one study, he says. That was also what Shaffer found in his groundbreaking research into teen suicides, which are five to seven times more likely to be committed by males.
Boys have another natural factor that makes them more likely to turn to aggression, he says. They are slower to learn verbal skills and tend to grab what they want.
Beyond body chemistry, can't we assign some blame to American culture and peer pressure?
Boys learn from other boys that being a man means being in control, says Dick Bathrick, who founded Men Stopping Violence in 1982 in Atlanta.
``You've got to be in control at work. You've got to be in control at home. You've got to be in control of your feelings,'' he says, according to this popular but ``very distorted concept of masculinity.''
When losing a job, or being rebuffed romantically, or having a spouse refuse to do what one says, men are more likely than women to become violent to regain control, Bathrick says.
Drive to Dominate
The drive to overpower women showed up in several school shootings. Last September in Bailey, Colorado, 53-year-old Duane Roger Morrison entered a high school, took six girls hostage and sexually assaulted them. When police showed up, he shot a 16- year-old girl dead before killing himself.
The following month, Carl Charles Roberts IV, 32, lined up girls at the West Nickel Mines Amish School and shot 10 of them, ages 6 to 13, before killing himself. Five of the girls died.
Seung Hui Cho's anger, like the shots he fired last week at Virginia Tech, seems to have been indiscriminate. His rantings target women but also accuse the rich and the world in general. It may mean much or nothing that he previously stalked two female students, became suicidal when rebuffed and launched his deadly spree last week by first shooting a young woman in her dorm. Cho killed 32 students and teachers in all before killing himself.
Groups such as Bathrick's have sprung up around the country to try to stop men from hurting the women in their lives. The organization also works to encourage non-violent men to challenge the misogyny and violence in others, Bathrick says.
In Washington, Men of Strength clubs in high schools steer teenagers toward healthy ideas of what manhood means, says Patrick Lemmon, executive director of Men Can Stop Rape, which sponsors the clubs. Not to mention the work of scout troops and boys' clubs.
No one claims an intervention of that sort could have stopped Cho, who may well have been psychotic.
But given the gross overrepresentation of men among those who kill, rape, molest and beat, you have to hope that more men become more outraged by the damage others of their gender inflict.
(Ann Woolner is a Bloomberg news columnist. The opinions expressed are her own.)