Vol. 8, No. 1, 2002 Page 5

Male aggression: inborn, not learned behavior

Men, in general, are far more aggressive than women. But are males born that way, or does society somehow "train" them to react more readily with angry words or fists? A study of rats indicates that nature, not nurture, holds the answer.

Jonathan Toot and colleagues introduced new male and female rats into an established colony, and measured the resulting aggression and stress of male and female rats. The researchers found that male intruders were attacked an average of 2.6 times, and received an average of 1.8 scars, during a 15-minute period. Female rats did not attack other rats, and were not attacked themselves.

The researchers measured levels of neurotransmitters in various brain regions of the rats, and found that male rats had significantly lower levels of dopamine and serotonin in areas of the amygdala, a brain region associated with aggression. In particular, they conclude, "decreased serotonin in the amygdala was associated with increases in aggressive behavior."

"The data show that males are involved in more fights than females," they say, "suggesting increased aggression is influenced by a Y-chromosomal effect that decreases amygdala serotonin." The researchers speculate that testosterone levels also contribute to males' increased aggression.

Daniel Ely, one of the researchers involved in the project, says the findings "open up the possibility of being able to identify the chromosome markers in high-risk males, and of trying to manipulate their levels of serotonin, through diet, exercise, or medication.a long way down the road, but this is the start."


"Sex differences in brain monoamines and aggression," Jonathan Toot, Gail Dunphy, and Daniel Ely, presentation to the American Physiological Society, October 17-20, 2001, Pittsburgh, PA. Address: Jonathan Toot, University of Akron, Department of Biology, Akron, OH 44325-3908. See also: "Fighting instinct is in men's genes," Lorna Martin, The Herald (UK), October 19, 2001.

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