Behavior: Sociobiology and Sex

O. Why do men go to war?

A. Because the women are watching.

This classic exchange may not be the last word on human aggression, but sociobiologists would admire the insight into male psychology. In their view, male displays and bravado—from antlers in deer and feather ruffling in birds to chest thumping in apes and humans—evolved as a reproductive strategy to impress females. Machismo is biologically based and says in effect: "I have good genes, let me mate."

But such male bluster works only if females allow it to work. Among many monogamous birds, a...

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