Gender: The Threatening Scent of Fertile Women

The 21-year-old woman was carefully trained not to flirt with anyone who came into the laboratory over the course of several months. She kept eye contact and conversation to a minimum. She never used makeup or perfume, kept her hair in a simple ponytail, and always wore jeans and a plain T-shirt.

Each of the young men thought she was simply a fellow student at Florida State University participating in the experiment, which ostensibly consisted of her and the man assembling a puzzle of Lego blocks. But the real experiment came later, when each man rated her attractiveness. Previous research had shown that a woman at the fertile stage of her menstrual cycle seems more attractive, and that same effect was observed here — but only when this woman was rated by a man who wasn't already involved with someone else.

The other guys, the ones in romantic relationships, rated her as significantly less attractive when she was at the peak stage of fertility, presumably because at some level they sensed she then posed the greatest threat to their long-term relationships.

... recent studies have found large changes in cues and behavior when a woman is at this stage of peak fertility. Lap dancers get much higher tips (unless they're taking birth-control pills that suppress ovulation, in which case their tips remain lower). The pitch of a woman's voice rises. Men rate her body odor as more attractive and respond with higher levels of testosterone.

“The fascinating thing about this time is that it flies under the radar of consciousness,” says Martie Haselton, a psychologist at U.C.L.A. “Women and men are affected by ovulation, but we don't have any idea that it is what is driving these substantial changes in our behavior. It makes it clear that we're much more like other mammals than we thought.”

At this peak-fertility stage, women are more interested in going to parties and dance clubs, and they dress more attractively (as judged by both men and women). Some women's attitudes toward their own partners also change, according to research by Dr. Haselton along with a U.C.L.A. colleague, Christina Larson, and Steven Gangestad of the University of New Mexico.

“Women who are in steady relationships with men who are not very sexually attractive — those who lack the human equivalent of the peacock's tail — suddenly start to notice other men and flirt,” Dr. Haselton said. “They are also more critical of their steady partners and feel less ‘one' with them on those few days before ovulation.” But that doesn't mean they're planning to walk out.

One safe way for both men and women to stay in a relationship is to avoid even looking at tempting alternatives, and there seem to be subtle mental mechanisms to stop the wandering eye, as Dr. Maner and colleagues at Florida State found in an experiment testing people's “attentional adhesion.”

The men and women in the experiment, after being primed with quick flashes of words like “lust” and “kiss,” were shown a series of photographs and other images. The single men and women in the study couldn't help staring at photographs of good-looking people of the opposite sex — their gaze would linger on these hot prospects even when they were supposed to be looking at a new image popping up elsewhere on the screen.

But the people who were already in relationships reacted differently. They looked away more quickly from the attractive faces. The subliminal priming with words related to sex apparently activated some unconscious protective mechanism: Tempt me not! I see nothing! I see nothing!

For more, see The Threatening Scent of Fertile Women by John Tierney, February 21, 2011 at NYTimes.com.