This past weekend, my friends were at a club. They found themselves less than thrilled with the scene; the sweaty, writhing crowd and overpriced drinks weren’t cutting it. They decided to leave, and roamed around in search of an open bar. Alas, it was almost 2:00 am and Boston was predictably dead. Not yet ready to call it a night, they cast about for something to do. “If only Aphrodite wasn’t with us, we’d go to Centerfold,” one of the guys grumbled. Aphrodite—the only girl in the group—perked up. She had heard about the infamous strip club, but had no idea our friends frequented the spot. She was intrigued, and it took little persuasion for her to agree to accompany the guys to the club.

I’ll leave what happened inside to your imagination, but it will suffice to say that, despite some initial hesitation and discomfort, Aphrodite ended up enjoying herself. The women were beautiful and the dancing was more sensual than explicitly sexual. Though the crowd was a little creepy (read: middle-aged and balding), they were generally respectful of the dancers. There was none of the leering, catcalling, and general griminess that she had expected from a strip club.

What surprised Aphrodite most, however, was how much she enjoyed watching the scantily clad women around her. She was unexpectedly attracted to the strippers, and was ogling their bodies just as much as the men surrounding her. When she relayed her story to me, she seemed uncomfortable to admit how aroused she had felt, and it was with bashful reluctance that she admitted to me that she allowed one of the guys to buy her a lap dance.

When I heard the story, I was enthralled. Kicking myself for deciding to stay in during what turned out to be such an eventful night, I couldn’t stop thinking about Aphrodite’s adventure. Why was it that my friend, who identifies as heterosexual, felt so attracted to female strippers? Why on earth did she acquiesce to receiving a lap dance, and why did she enjoy it?!

And to speak more generally, why is it more socially acceptable for straight women to have feelings of homoerotic attraction than it is for straight men? At parties, it is acceptable for drunk girls to playfully make out with each other, but that’s hardly the case for guys. It’s more acceptable for female celebrities to express same-sex attraction (remember Britney and Madonna’s infamous VMA kiss?). In an interview with British Cosmopolitan, Christina Aguilera was quoted as saying: “I think women are such sensual beings. And, I mean, I’m attracted to men ultimately—I’m married and I love my husband and I love what we do together, but honestly? If I had the choice between viewing a naked man or a naked woman, I’d choose the woman. We’re just naturally sexier and more beautiful to look at.” Though this is somewhat scandalous, it is much more acceptable than if straight male celebrity had said that he enjoyed looking at naked men. Similarly, if a situation arose where a group of my female friends decided to go to a male strip club, I can say with certainty that none of my heterosexual male friends would agree to come, let alone receive a lap dance. Is this bias due to socialization? Have we become more accepting of lesbian tendencies because of the culture in which we’ve been raised? Is this just another manifestation of the way women are objectified and sexualized in our society, or is there a biological explanation for this difference?

A team of researchers at Northwestern University tackled this question, publishing their results in a paper called “A Sex Difference in the Specificity of Sexual Arousal” (Chivers et al, 2004). In this study, researchers exposed both men and women video clips depicting both heterosexual and homosexual sexual behavior, and recorded their subjective arousal (by means of a self-report, using the Kinsey sexual fantasy scale) and physiological arousal. Psychological arousal was measured in males by measuring changes in the circumference of the penis, and in women by measuring the change in “vaginal pulse amplitude”, which measures female arousal. Moreover, the study included post-operative male-female transsexuals, in order to eliminate the possibility that the differences observed between men and women were due to the way genital arousal was measured, and ensured that they truly reflected differences in arousal.

What researchers found was that the relationship between sexual arousal and sexual orientation differs fundamentally between men and women. The data for men showed a strong correlation between male arousal and their stated sexual orientation: straight men were significantly more aroused while watching videos showing female-male and female-female sexual behavior, while gay men were significantly more aroused watching male-male sex. The data for women told a different story. Both heterosexual and homosexual women experienced strong genital arousal to both male and female sexual stimuli. In other words, both homosexual and heterosexual women were significantly aroused watching male-female sex as well as female-female sex. In other words, lesbians can be aroused by watching straight sex, and straight women can be turned on by watching lesbian sex. According to the authors of the study, “female sexuality seems generally to be more flexible than male sexuality.”

This study has been repeated (with different variations) several times since 2004 (Bailey, 2009; Chivers, 2005; Chivers, Rieger, Latty, & Bailey, 2004; Lippa, 2006, 2007; Suschinsky, Lalumiere, & Chivers, 2009). In these studies, researchers used different methods to generate arousal, from video clips to audio clips to pictures of swimsuit models. Regardless of methodological variations, each reports the same thing: women become highly aroused watching both lesbian and heterosexual sexual behavior, regardless of their sexual orientation.

The conclusion at which all of these studies have arrived is that men exhibit “category specific” sexual responses. This is because they experience both genital and psychological sexual arousal when they watch films depicting their preferred sex, but not when they watch films depicting the other sex. Women, however, do not show a ‘‘category-specific’’ genital response to erotic stimuli, and as a result heterosexual and lesbian women are indistinguishable in terms of their responses to heterosexual versus lesbian erotica. One of the reasons that men are category-specific while women are not is that homosexuality is much more likely to be due to genetics in men than it is for women. There is more evidence that for men, sexual orientation is due to biological contributions, whereas there is little evidence that female homosexuality is due to biological factors (Mustanski et al., 2003; Diamond, 2003). Furthermore, there are very few women who are exclusively homosexual; most lesbian women have had heterosexual sex and relationships with men. This makes sense given the difference in category specificity examined above; men are so reliable in their patterns of sexual arousal that researchers can accurately predict a man’s sexual orientation by measuring his genital arousal in the lab. This is probably because male sexual orientation is coded in their genetic makeup; it’s biological. Women’s sexual orientation cannot be predicted using biological indicators; it is instead determined mainly by socialization (nurture, instead of nature), and is therefore more fluid.

You might be wondering why you don’t see more women hooking up with each other, since they clearly become aroused watching other naked women. According to a study done by Laumann, Gagnon, Michael, & Michaels, “despite their capacity to become sexually aroused by both male and female sexual stimuli, women do not have higher rates of same-sex sexual activity than men” (1994). So, even though straight girls can appreciate and even become turned on by naked girls, they still prefer to have sex with men.

The Chivers study addresses this issue as well, and Aphrodite would be comforted to know that “a self-identified heterosexual woman would be mistaken to question her sexual identity because she was aroused watching female–female erotica; most heterosexual women experience such arousal.” The authors also comment on male category specificity, adding “a self-identified heterosexual man who experienced substantial arousal to male–male erotica, however, would be statistically justified in reconsidering his sexual identity” (Chivers 2004).

All things considered, it’s completely normal—even expected—that straight women are turned on by other women. Aphrodite can relax—she can go to strip clubs and enjoy lap dances without having to doubt her sexual orientation. Maybe next time I’ll even join her.