The existence of female competition may seem obvious to anyone who has been in a high-school cafeteria or a singles bar, but analyzing it has been difficult because it tends be more subtle and indirect (and a lot less violent) than the male variety. Now that researchers have been looking more closely, they say that this “intrasexual competition” is the most important factor explaining the pressures that young women feel to meet standards of sexual conduct and physical appearance.

The old doubts about female competitiveness derived partly from an evolutionary analysis of the reproductive odds in ancient polygynous societies in which some men were left single because dominant males had multiple wives. So men had to compete to have a chance of reproducing, whereas virtually all women were assured of it.

But even in those societies, women were not passive trophies for victorious males. They had their own incentives to compete with one another for more desirable partners and more resources for their children. And now that most people live in monogamous societies, most women face the same odds as men. In fact, they face tougher odds in some places, like the many college campuses with more women than men.

To see how female students react to a rival, researchers brought pairs of them into a laboratory at McMaster University for what was ostensibly a discussion about female friendships. But the real experiment began when another young woman entered the room asking where to find one of the researchers.

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.

So the short skirt is threatening but the jeans are not?

Maybe Kim Kardashian needs to go out more in jeans...