MEAN GIRLS: A Cold War Fought By Women.

This woman had been chosen by the researchers, Tracy Vaillancourt and Aanchal Sharma, because she “embodied qualities considered attractive from an evolutionary perspective,” meaning a “low waist-to-hip ratio, clear skin, large breasts.” Sometimes, she wore a T-shirt and jeans, other times a tightfitting, low-cut blouse and short skirt.

In jeans, she attracted little notice and no negative comments from the students, whose reactions were being secretly recorded during the encounter and after the woman left the room. But when she wore the other outfit, virtually all the students reacted with hostility.

They stared at her, looked her up and down, rolled their eyes and sometimes showed outright anger. One asked her in disgust, “What the [expletive] is that?”

Most of the aggression, though, happened after she left the room. . . . “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.

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