A small Canadian study at McMaster University showed that women are more likely to be shrewdly hostile towards women who they regard as promiscuous. Tracy Vaillancourt, lead author of the study and psychologist at University of Ottawa, explained that this reaction is a form of unfriendliness that women often use, called “indirect aggression” — more commonly known as bitchiness or slut-shaming.

To experimentally gauge this behavior, Vaillancourt and Ph.D student, Aanchal Sharma, observed 86 straight women who were paired off and made to think that they were participating in a study about female friendship. At some point a plainly dressed yet attractive, blonde woman with her hair in a bun — the “conservative confederate” — interrupted half of these pairs. The same woman also disrupted the other half, but was dressed in a low-cut blouse, short skirt, boots; and with her hair down. The experimenters called this less-traditional alter ego the “sexy confederate.”

Participants’ reactions were assessed and scored according to a “bitchiness” scale. Revealing body language such as eye rolling, glaring, looking the confederate up and down, and laughing sarcastically added to the score. The participants and scorers were approximately the same age range, between 20 and 25 years old.

One participant went as far as saying that the sexy confederate was dressed that way in order to have sex with the professor, Vaillancourt told The Atlantic. In another case, the sexy confederate hadn’t even left the room before someone burst out with a rude comment. Altogether, women’s reactions toward the sexy confederate scored much higher on the bitchiness scale in comparison to how they regarded the conservative confederate. Bitchiness also scored higher when the women were among friends rather than strangers.

Vaillancourt explained that men also exercise indirect aggression but not as often as women do, according to CBS News. "When we aggress against somebody, we do it indirectly. When men aggress against others, a lot of times it's direct, it's verbal," she said. "We exclude people from the peer group, we give the silent treatment … We'll spread rumors about the person, so we'll disparage their appearance, we'll suggest that they're promiscuous, those sorts of things."

Vaillancourt emphasized the importance of this research — it addresses a common social issue that hasn’t been scientifically studied up until this point. This behavior should be regarded as more than just mere pettiness because indirect aggression is an effective way of inflicting emotional harm. "We needed to have science to support what our anecdotal evidence is, and certainly I can appreciate that it is a little off-putting that women behave poorly, we don't like to be told the truth," she told CBS. "But it's important because we have girls who take their lives over this — about the way they're treated."

The wide use of online social networks has apparently allowed this form of bullying, such as slut-shaming or insults about a person's physique, to become even more rampant, making Vaillancourt’s work all the more relevant.

In another experiment from the same study, Vaillancourt showed a new group of women photographs of both the conservative and sexy confederate, as well as a photo of the sexy confederate that was doctored to make her look overweight. They found that women preferred not to be friends with the sexy confederate nor with the photoshopped version. They were also less inclined to introduce their boyfriends to her, or let their boyfriends spend time with either version of the sexy woman.

Altogether, women were three times more likely to introduce the buttoned-up, confederate woman to their boyfriend than the thin sexy one. Vaillancourt told The Atlantic that this difference revealed how women “are threatened by, disapprove of, and punish women who appear and/or act promiscuous.”

A study in the Review of General Psychology described the regard that women have towards other women’s apparent promiscuity as a form of cultural suppression of female sexuality that is mostly carried out by women. “The view that men suppress female sexuality received hardly any support and is flatly contradicted by some findings,” authors, Roy Baumeister and Jean Twenge, concluded. “Instead, the evidence favors the view that women have worked to stifle each other’s sexuality because sex is a limited resource that women use to negotiate with men, and scarcity gives women an advantage.”

Source: Vaillancourt T, Charma A. Intolerance of sexy peers: intrasexual competition among women. Aggressive Behavior. 2013.