Jesse Bering investigates why teenage girls are so cruel to each other:

Although most researchers acknowledge the somewhat speculative nature of evolutionary arguments in this area, female social aggression among reproductively viable females is usually interpreted as a form of mate competition.

Hess and Hagen, for example, suggest that the sex differences uncovered in their study would likely have been even more pronounced in a younger group of participants. Evolutionarily, historically and cross-culturally, they point out, girls in the fifteen- to nineteen-year-old range would be most actively competing for mates. Thus, anything that would sabotage another females’ image as a desirable reproductive partner, such as commenting on her promiscuity, physical appearance or some other aberrant or quirky traits, tends to be the stuff of virile gossip. Also, the degree of bitchiness should then demonstrate a sort of bell-shaped curve over the female life course. On the surface this seems mostly true. Anecdotally, I can’t think of a single postmenopausal woman who seems hell-bent on undermining another woman’s dating life...

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