The Pill — a term that refers to any one of the many different kinds of birth-control pills taken by women to ward off pregnancy — has been a boon to modern sexuality. Without worry of pregnancy before a woman is ready, it allows two people to enjoy sex for the pleasure of it, not for its procreation value.

But like any medication, the pill always has had side effects. In some women, those side effects are unbearable, and so another birth control method must be tried.

According to new research, the side effects also extend to a woman’s general enjoyment of sex itself. The new study, led by Craig Roberts and colleagues, conducted a survey of 2,519 women who had at least one child. The researchers asked the women about their relationship quality — including sex — with their child’s biological father.

What did they find?

The study found, somewhat ironically, that women who took the pill were less sexually satisfied. Not only that, they also said that they found their partners less attractive, and were more likely to be the one to initiate an eventual separation from their partner.

I say “ironically” because one of the points of taking the pill is to experience and enjoy as much as sex as one wants. Yet pill users appear to enjoy sex less than non-pill users.

On the flip side, the researchers found that for women who were on the pill, relationships with their man lasted on average 2 years longer than for women who weren’t on the pill. They also said that they liked how their partner was more reliable and caring, and were more satisfied with these paternal traits.

How could this be?

The mechanism for the finding is by no means certain, but Roberts speculated that it could have something to do with a set of genes called the major histocompatibility complex (MHC), a key component of a person’s immune system. This set of genes also has an important evolutionary role in guiding people to find mates who are genetically dissimilar, because genetic diversity within their offspring’s MHC will lead to an increased chance of surviving novel diseases and environments. “Women tend to find genetically dissimilar men attractive because resulting babies will more likely be healthy,” said Roberts. “It’s part of the subconscious ‘chemistry’ of attraction between men and women.”

Roberts suggest that since women on the pill don’t experience the typical hormonal level shifts that normal women experience, they may be more attracted to men who are more caring and compatible with their MHC genes. As the Guardian notes, “This may lead to lower sexual satisfaction but will land them with more reliable men who, ultimately, make them more generally satisfied.”

It’s an interesting finding that needs replication to ensure it’s a robust one. If true, it’s one more factor to add into the equation of deciding what kind of birth control is right for you.

The new research was published in the Oct. 12 issue of Proceedings of the Royal Society.

Read the full article: Pill may make sex less satisfying for women, but relationships last longer

The Pill May Dampen Women’s Sexuality, But Increases Relationship Strength