Heavyweight champion Mike Tyson is famous for fighting both inside and outside the ring. Boys who box may do the same, according to a study, not because they're inherently violent but because combative sports seem to make for combative kids.

For two years, researchers at the University of Bergen in Norway followed nearly 500 preteen boys who took up boxing, wrestling, weight lifting and martial arts. These boys, on the whole, began to start fights, steal and skip school far more often than their peers. Compared with nonathletes or players of milder sports like soccer, "power sport" players were about five times as likely to be antisocial, says researcher Inger Endresen.

The researchers emphasize that the high rates of bad behavior aren't because power sports simply attract rowdy kids, as many coaches claim. For the first time, bullying experts observed boys both before and after they took up power sports, so it became clear that aggressiveness followed sports, instead of the other way around.

So what's going on? It's probably not the actual sport but the macho attitudes and ideals prevalent in the sports' culture that affect a boy's outlook. After all, says Endresen, even contact-free weight lifting is linked to greater levels of aggression.