Social factors undoubtedly account for a part of the differences. But in a study published in 2007 psychologist Raymond Baillargeon of the University of Montreal and his colleagues reveal that as early as the age of 17 months, 5 percent of boys but only 1 percent of girls engage in frequent physical aggression, such as kicking and biting. What is more, this gap does not widen between 17 and 29 months, as might be expected if environmental influences such as socialization by parents were to blame. These findings suggest that biological factorssuch as the effects of testosterone on brain functioncontribute to sex differences in violent behavior.
Bolstering this hypothesis is the fact that males are the more belligerent sex in virtually all mammalian species that biologists have studied. Even the one marked exception to this trendthe spotted (laughing) hyenamay prove the rule. The female hyena, which is more physically aggressive than her male counterpart, has higher testosterone levels than the male does.