Why are girls often so cruel to each other?

Female gossip-based attacks are largely a function of competition for men.

Via Jesse Bering‘s excellent book, Why Is the Penis Shaped Like That?: And Other Reflections on Being Human:

Although most researchers acknowledge the speculative nature of evolutionary arguments in this area, 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 femaleâs 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.

The age speculation was validated in another study. Amount of gossip is tied to women’s age/fertility:

In the current study it was investigated whether age differences in womenâs tendency to gossip exist, and whether these could be accounted for by self-reported mate value. It was expected that younger women would report a higher tendency to gossip after reading a scenario in which a romantic rival was introduced, because they are at an age when competition for mates is salient and often intense. Moreover, it was hypothesized that this higher tendency to gossip would be attributable to these womenâs higher mate value, since younger women have a higher reproductive capacity than older women. The results confirm these expectations: age differences in womenâs tendency to gossip disappeared when controlling for mate value. Discussion focuses on the interpretation and implications of these results.

Source: “Age differences in womenâs tendency to gossip are mediated by their mate value” from Personality and Individual Differences, Volume 52, Issue 1, January 2012, Pages 106â109