Since Kinsey's pioneering work during the 1950s, psychologists have continued to expand their study of sexual orientation. In fact, a number of interesting findings have surfaced in the scientific literature. Heterosexual, homosexual, bisexual, pansexual are all terms that one might use to refer to their sexual orientation. Bailey (2009) defines sexual orientation as a compass that directs sexuality (i.e., sexual feelings, desire, arousal, fantasy and attraction). He is very careful to draw a line and distinguish between sexual behavior pattern (one's history of sexual and romantic relationships with men vs. women), private sexual identity (how you view yourself) and sexual preference (the choice you make regarding the sex of an erotic partner).
1) Sexual orientation and gender differences in arousal: Arousal arguably consists of many factors, although most sexuality researchers discuss arousal as consisting of two parts: physical arousal and subjective arousal. Physical arousal typically involves an erection in a man and vaginal engorgement and lubrication in a woman. Subjective arousal has to do with how aroused one "feels." For men, there is generally concordance, or agreement between their reports of subjective arousal and measures of their physical arousal. But this is not true for women.
Chivers, Rieger, Latty, and Bailey (2004) conducted research where they presented different kinds of erotic stimuli to homosexual and heterosexual men. The research included a measure of the male's genital response (an objective measure of physical arousal) and then asked them to describe what was arousing to them (to measure subjective arousal). Results from these studies show that men become physically and subjectively aroused by what they orient to. This simply means that a man who calls himself heterosexual will say that he becomes aroused by images that contain women whereas a man who calls himself homosexual will say that he becomes aroused by images that contain men. Additionally, a man who calls himself heterosexual achieves a penile erection when shown images that contain women whereas a man who calls himself homosexual achieves a penile erection when shown images that contain men. We could thus say that male arousal patterns seem to be parallel, meaning that physical and subjective arousal line up. This has led Bailey (2009) to suggest that a man's sexual arousal pattern is his sexual orientation. This however, is not the case for women.
Results from the Chivers, et. al. (2004) study yielded very different results for women. Contrary to men, women become subjectively aroused by what they oriented to, but physically aroused to images of both men and women. This simply means that a woman who calls herself heterosexual, will say that she becomes aroused by images that contain men; whereas a woman who calls herself lesbian, will say that she becomes aroused by images that contain women. However, when we consider her physiological arousal (vaginal engorgement) we find that she became physically aroused to all images presented to her. Thus, we could say that women have a non-specific sexual arousal pattern (meaning that they do not physically arouse to a specific category, a.k.a. gender). This research suggests that perhaps female sexual orientation is not directed at a specific gender.
2) Sexual orientation and gender difference in desire: In addition to the gender differences noted above in arousal there are also gender differences in desire. In fact, Lisa Diamond, Ph.D. has offered both theoretical models (e.g., 2003; 2004; 2005) and research data (e.g., 2003; 2005) supporting the idea that changes in sexual orientation are not uncommon among women. For example, Diamond (2003) found that over the course of a five-year period, 25% of the women in her study who initially identified as lesbian or bisexual changed their sexual identity over time. This suggests that women may change their sexual orientation over time. Diamond (2005) also theorized women may be more likely than men to fall in love with an individual, regardless of gender/sex.
Diamond further suggests that there are at least three different types of lesbians: 1) Fluid Lesbians, 2) Nonlesbians, and 3) Stable Lesbians. She describes fluid lesbians as women who alternate between changing their self-reported sexual orientation over time (meaning that they might at one point call themselves lesbian yet at another point in time call themselves heterosexual). Non-lesbians are defined as women who do not adopt a label even though they report being attracted to other women and acting on these attractions. Finally, stable lesbians are women who consistently characterize their sexual orientation as lesbian. Thus, women's sexual desire may be more fluid than men's and may be more dependent on romantic attachment than on attraction to a particular sex. Women may be more likely to fall in love with an individual person, regardless of their gender, whereas men may be more likely to fall in love with (or be attracted to) a person based on their gender.