The Tricky Chemistry of Attraction

Much of the attraction between the sexes is chemistry. New studies suggest that when women use hormonal contraceptives, such as birth-control pills, it disrupts some of these chemical signals, affecting their attractiveness to men and women’s own preferences for romantic partners. The type of man a woman is drawn to is known to change during her monthly cycle—when a woman is fertile, for instance, she might look for a man with more masculine features. Taking the pill or another type of hormonal contraceptive upends this natural dynamic, making less-masculine men seem more attractive, according to a small but growing body of evidence. The findings have led researchers to wonder about the implications for partner choice, relationship quality and even the health of the children produced by these partnerships. Evolutionary psychologists and biologists have long been interested in factors that lead to people’s choice of mates. One influential study in the 1990s, dubbed the T-shirt study, asked women about their attraction to members of the opposite sex by smelling the men’s T-shirts. The findings showed that humans, like many other animals, transmit and recognize information pertinent to sexual attraction through chemical odors known as pheromones.

The study also showed that women seemed to prefer the scents of men whose immune systems were most different from the women’s own immune-system genes known as MHC. The family of genes permit a person’s body to recognize which bacteria are foreign invaders and to provide protection from those bugs. Evolutionarily, scientists believe, children should be healthier if their parents’ MHC genes vary, because the offspring will be protected from more pathogens. More than 92 million prescriptions for hormonal contraceptives, including pills, patches and injections, were filled last year in the U.S., according to data-tracker IMS Health. Researchers say their aim isn’t to scare or stop women from taking hormonal contraceptives. “We just want to know what we’re doing” by taking the pill, says Alexandra Alvergne, a researcher in biological anthropology at University College London in the U.K. “If there is a risk it affects our romantic life and the health status of our children, we want to know.” Dr. Alvergne last year published a review detailing the existing literature on the topic in the journal Trends in Ecology and Evolution.

Both men’s and women’s preferences in mates shift when a woman is ovulating, the period when she is fertile, research has shown. Some studies have tracked women’s responses to photos of different men, while other studies have interviewed women about their feelings for men over several weeks. Among the conclusions: When women are ovulating, they tend to be drawn to men with greater facial symmetry and more signals of masculinity, such as muscle tone, a more masculine voice and dominant behaviors.

The women also seemed to be particularly attuned to MHC-gene diversity. From an evolutionary perspective, these signals are supposed to indicate that men are more fertile and have better genes to confer to offspring. Women tend to exhibit subtle cues when they are ovulating, and men tend to find them more attractive at this time. Women try to look more attractive, perhaps by wearing tighter or more revealing clothing, says Martie Haselton, a communications and psychology professor at the University of California, Los Angeles. Research on this includes studies in which photos that showed women’s clothing choices at different times of the month were shown to groups of judges. Women also emit chemical signals that they are fertile; researchers have measured various body odors, says Dr. Haselton, who has a paper on men’s ability to detect ovulation coming out in the journal Current Directions in Psychological Science.

Very interesting. I have insane chemistry with the wrong people! I find it impossible to control.

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