Katy Perry's "I Kissed A Girl" made waves on the radio in 2008 when she sang about being promiscuous with another girl. The lyrics "You're my experimental game/Just human nature" allude to the flexibility that exists within female sexuality, where it's seen as more socially acceptable for women to “experiment” with same-sex hookups. It seems Katy Perry's got it right — women have been evolutionarily designed to have lesbian encounters.

A recent study published in the journal Biological Reviews suggests women's sexuality has evolved to be more fluid than men's as a mechanism to reduce conflict and tension among co-wives in polygynous marriages.

Sexual fluidity encompasses how we identify our sexual preferences, such as straight or gay; our actual sexual behavior; sexual thoughts and fantasies; and genital or brain responses to sexual stimuli. Evolutionarily speaking, sexual fluidity kept women mating and reproducing offspring with their husbands, even when their sexual preference was not strictly heterosexual.

Dr. Satoshi Kanazawa, lead author of the study and an evolutionary psychologist at the London School of Economics and Political Science, explains genital and brain responses are the most objective and accurate measure of sexual orientation. In other words, genital and brain sexual arousal patterns (measured via fMRI scans), are seen as clear indicators of our sexual preferences. Scientists measure sexual preferences by looking at whether people engage in nonexclusive relationships or in unconventional sexual behavior or whether their sexual preference changes over time.

To determine the differences between sexuality fluidity in men and women, he analyzed the National Longitudinal Study of Adolescent Health (Add Health), which measured sexual orientation of American youths of both genders in four “waves” over a span of  10 years. Kanazawa measured self-identified sexual feelings among a given set of labels, including 100 percent straight, mostly straight, bisexual, mostly gay, and 100 percent gay when participants were between 18 and 28 in wave III. In Wave IV, when respondents were between 25 and 34, they were asked to describe sexual identity.

The findings revealed when it came to sexuality, women were more fluid in each of the proposed ways. It confirmed women who experience increased levels of sexual fluidity have a larger number of children, and women who experience marriage or parenthood early in adult life also experience increased levels of sexual fluidity. This suggests women’s sexual fluidity may have been evolutionarily selected as a coping mechanism for polygynous marriages, even though humans have only been mildly polygynous throughout evolutionary history. Typically, these marriages are often characterized by conflict and tension among co-wives.

“I propose that occasional sex among co-wives may have reduced such conflict and tension, and increased their reproductive success. Female sexual fluidity may have evolved as an adaptation to facilitate it,” wrote Kanazawa, in the paper.

Perhaps women-on-women sexual escapades once had an evolutionary advantage. This theory also introduces the notion that women may not be sexually oriented in the same sense as men. So, rather than being straight or gay, the person women are sexually attracted to depends on their particular partner, reproductive status, and other circumstances.

A similar 2015 study found straight women turned on by attractive women are either “bisexual or gay.” Researchers believe women's sexual preferences tend to be a gray area. Straight women were strongly sexually aroused by videos of both attractive men and women, even if they chose men as their sexual preference. The truth is straight women, not just lesbians, ogle at beautiful women.

The distinction between admiration and same-sex fantasies should be noted, but not generalized. Women can think other women are beautiful, and not have sexual fantasies about her. Or, some women can do both.

The evolutionary advantage to same-sex hookups in women may also lend itself to explaining many other puzzles in human sexuality, including male arousal to lesbian sex, and menstrual synchrony, where our girlfriends’ menstrual cycles all align together.

So, science explained to us all why Katy Perry kissed a girl, and liked it.

Source: Kanazawa S. Possible evolutionary origins of human female sexual fluidity. Biological Reviews. 2016.