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.