The pill increases the sex hormone binding globulin, a protein that makes testosterone inactive. That could be a problem because testosterone plays an important role in sexual motivation for both genders. It can also suppress estrogen synthesis which can affect a woman’s sexual response. “This combination of keeping estrogen suppressed and making testosterone inactive really decreases not only sexual motivation, like actually wanting to have sex, but also the feelings of sexiness that for women, are really important in terms of our libido,” Hill says. The good news is that new types of progestins, the artificial hormone found in the pill, might have less of an effect on libido than ones of the past, so it’s something to ask your doctor about if you’re noticing that you’re disinterested in sex while taking the pill.
“The side effects that make women quit the pill are usually mood-related,” Hill says. According to research, the pill can cause issues with anxiety and depression. The reason: The pill lowers women’s levels of the calming neurosteroid allopregnanolone, which can decrease the brain's ability to help a person slow themselves down. On the flip side, some women say that the pill elevates their mood and alleviates the negative psychological effects of PMS, such as sadness or feeling stressed out. “There's been a number of studies demonstrating women who had really severe PMS, and the pill can really be helpful to them,” says Hill. If you think the pill might be affecting your moods in a negative way, talk to your doctor about changing to a different pill or method of birth control altogether.
According to research cited in the book, naturally-cycling women tend to be more in tune with traditional masculine attributes—social dominance, a strong jawline, and broad shoulders, for example—than women who are on the pill because their estrogen levels aren’t fluctuating as much. And it could affect their mate choice. “Pill takers were choosing partners with more feminine faces and reported being more satisfied with some of the non-physical aspects of their partners, things like intelligence and financial provisioning,” Hill says. In fact, women who choose their partners while they’re on the pill actually have a lower divorce rate than women who cycle naturally, perhaps because they picked their partner for their long-term attributes.
Sounds all good, but potential problems arise when those women stop taking the pill. “All of a sudden, they're not attracted to their partner,” says Hill. “For the pill-takers, when they did get divorced, it was overwhelmingly the female who initiated it,” Hill says, the idea being that taking artificial hormones could have affected their mate choice. Still, she assures that this is not always the case. “I don’t want anyone to start overthinking their perfectly happy relationships just because of what their contraceptive status was at the time that they chose their partner,” she says. “It's just one of those things for women to keep in mind.”
One of Hill’s graduate students found in her preliminary research that pill-taking women performed worse than natural-cycling women on a difficult exam, and that they were more likely to give up on unsolvable word puzzles. This is consistent with research finding that pill-taking women have structural differences in the hippocampus (believed to stem from excessive cortisol signaling) that can impair the ability to regulate learning and memory.