Most of the aggression, though, happened after she left the room. Then the students laughed about her and impugned her motives. One student suggested that she dressed that way in order to have sex with a professor. Another said that her breasts “were about to pop out.”

The results of the experiment jibe with evidence that this “mean girl” form of indirect aggression is used more by adolescents and young women than by older women, who have less incentive to handicap rivals once they marry. Other studies have shown that the more attractive an adolescent girl or woman is, the more likely she is to become a target for indirect aggression from her female peers.

“Women are indeed very capable of aggressing against others, especially women they perceive as rivals,” said Dr. Vaillancourt, now a psychologist at the University of Ottawa. “The research also shows that suppression of female sexuality is by women, not necessarily by men.”

Stigmatizing female promiscuity — a.k.a. slut-shaming — has often been blamed on men, who have a Darwinian incentive to discourage their spouses from straying. But they also have a Darwinian incentive to encourage other women to be promiscuous. Dr. Vaillancourt said the experiment and other research suggest the stigma is enforced mainly by women.

“Sex is coveted by men,” she said. “Accordingly, women limit access as a way of maintaining advantage in the negotiation of this resource. Women who make sex too readily available compromise the power-holding position of the group, which is why many women are particularly intolerant of women who are, or seem to be, promiscuous.”

Indirect aggression can take a psychological toll on women who are ostracized or feel pressured to meet impossible standards, like the vogue of thin bodies in many modern societies. Studies have shown that women’s ideal body shape is to be thinner than average — and thinner than what men consider the ideal shape to be. This pressure is frequently blamed on the ultrathin female role models featured in magazines and on television, but Christopher J. Ferguson and other researchers say that it’s mainly the result of competition with their peers, not media images.

“To a large degree the media reflects trends that are going on in society, not creates them,” said Dr. Ferguson, a psychologist at Stetson University. He found that women’s dissatisfaction with their bodies did not correlate with what they watched on television at home. Nor were they influenced by TV programs shown in laboratory experiments: Watching the svelte actresses on “Scrubs” induced no more feelings of inferiority than watching the not-so-svelte star of “Roseanne.”