Birth control pills don’t merely prevent women from becoming pregnant; they may also disrupt the way they decide who to marry or choose to partner with. Research at Stirling University in Scotland shows that the hormones in these pills may cause women to choose men they would not find attractive if they weren’t taking the pill.

The pills lower testosterone levels in women and, as a result, make them more attracted to men with lower testosterone. They may also interfere with women’s instinctive preferences for men who are more genetically suitable to father their children.

In one study, researchers found that contraceptive pills shifted women’s odor preferences in finding men attractive. Normally, women’s partner choices are influenced by body odor; therefore, they tend to be attracted to men with a dissimilar genetic make-up, which helps maintain genetic diversity in their children. Genes in the body’s Major Histocompatibility Complex (MHC) help build proteins for the immune system and affect body odor during interactions with skin bacteria. In this way, genes help determine which individuals find each other attractive.

Craig Roberts, a researcher and professor says: “The results showed that the preferences of women who began using the contraceptive pill shifted towards men with genetically similar odors.

“Not only could MHC-similarity in couples lead to fertility problems but it could ultimately lead to the breakdown of relationships when women stop using the contraceptive pill, as odor perception plays a significant role in maintaining attraction to partners.”