But a new Northwestern University study boosts the relatively limited research on women's sexuality with a surprisingly different finding regarding women's sexual arousal.
In contrast to men, both heterosexual and lesbian women tend to become sexually aroused by both male and female erotica, and, thus, have a bisexual arousal pattern.
"These findings likely represent a fundamental difference between men's and women's brains and have important implications for understanding how sexual orientation development differs between men and women," said J. Michael Bailey, professor and chair of psychology at Northwestern and senior researcher of the study "A Sex Difference in the Specificity of Sexual Arousal." The study is forthcoming in the journal Psychological Science.
Bailey's main research focus has been on the genetics and environment of sexual orientation, and he is one of the principal investigators of a widely cited study that concludes that genes influence male homosexuality.
As in many areas of sexuality, research on women's sexual arousal patterns has lagged far behind men's, but the scant research on the subject does hint that, compared with men, women's sexual arousal patterns may be less tightly connected to their sexual orientation.
The Northwestern study strongly suggests this is true. The Northwestern researchers measured the psychological and physiological sexual arousal in homosexual and heterosexual men and women as they watched erotic films. There were three types of erotic films: those featuring only men, those featuring only women and those featuring male and female couples. As with previous research, the researchers found that men responded consistent with their sexual orientations. In contrast, both homosexual and heterosexual women showed a bisexual pattern of psychological as well as genital arousal. That is, heterosexual women were just as sexually aroused by watching female stimuli as by watching male stimuli, even though they prefer having sex with men rather than women.
"In fact, the large majority of women in contemporary Western societies have sex exclusively with men," said Meredith Chivers, a Ph.D. candidate in clinical psychology at Northwestern University, a psychology intern at the Centre for Addiction and Mental Health and the study's first author. "But I have long suspected that women's sexuality is very different from men's, and this study scientifically demonstrates one way this is so."
The study's results mesh with current research showing that women's sexuality demonstrates increased flexibility relative to men in other areas besides sexual orientation, according to Chivers.
"Taken together, these results suggest that women's sexuality differs from men and emphasize the need for researchers to develop a model of the development and organization of female sexuality independent from models of male sexuality," she said.
The study's four authors include Bailey and three graduate students in Northwestern's psychology department, Chivers, Gerulf Rieger and Elizabeth Latty.
"Since most women seem capable of sexual arousal to both sexes, why do they choose one or the other?" Bailey asked. "Probably for reasons other than sexual arousal."
Sexual arousal is the emotional and physical response to sexual stimuli, including erotica or actual people. It has been known since the early 1960s that homosexual and heterosexual men respond in specific but opposite ways to sexual stimuli depicting men and women. Films provoke the greatest sexual response, and films of men having sex with men or of women having sex with women provoke the largest differences between homosexual and heterosexual men. That is because the same-sex films offer clear-cut results, whereas watching heterosexual sex could be exciting to both homosexual and heterosexual men, but for different reasons.
Typically, men experience genital arousal and psychological sexual arousal when they watch films depicting their preferred sex, but not when they watch films depicting the other sex. Men's specific pattern of sexual arousal is such a reliable fact that genital arousal can be used to assess men's sexual preferences. Even gay men who deny their own homosexuality will become more sexually aroused by male sexual stimuli than by female stimuli.
"The fact that women's sexual arousal patterns are not all predicted by their sexual orientations suggests that men's and women's minds and brains are very different," Bailey said.
To rule out the possibility that the differences between men's and women's genital sexual arousal patterns might be due to the different ways that genital arousal is measured in men and women, the Northwestern researchers identified a subset of subjects: postoperative transsexuals who began life as men but had surgery to construct artificial vaginas.
In a sense, those transsexuals have the brains of men but the genitals of women. Their psychological and genital arousal patterns matched those of men -- those who like men were more aroused by male stimuli and those who like women were more aroused by the female stimuli -- even though their genital arousal was measured in the same way women's was.
"This shows that the sex difference that we found is real and almost certainly due to a sex difference in the brain," said Bailey.