The parameters for bisexuality can be difficult to establish. At present "no scien­tific or popular consensus exists on the precise cluster of experiences that qualify

an individual as lesbian, gay, or bisexual [or heterosexual] instead of just curious, confused, or experimenting" (Diamond, 2008a, pp. 26-27). As seen in Table 9.1, 3.5% of women and 1.1% of men identify themselves as bisexual (Chandra et al., 2011). Even when people consider themselves bisexual, their bisexuality is often unknown to others because of the common assumption that people are either straight or gay, based on the sex of their current partner (Plato, 2008).

Research about bisexuality is quite limited, but what does the available research tell us? One study of men indicated that sexual arousal in self-identified bisexual men is associated with a unique and specific pattern. The researchers measured the subjective—how aroused they felt—and erectile responses of bisexual, homosexual, and heterosexual men while they watched various sexual videos— male-male, male-female, and a man having sex with both a woman and another man. As anticipated, homosexual and heterosexual men demonstrated arousal, respectively, to male-male videos and male­ female videos. Bisexual men were aroused by both gay and straight

videos, but their arousal by the video of a man engaging in sex with both a man and a woman was significantly higher than gay and straight men’s arousal by the same video (Cerny & Janssen, 2011).

Several research studies have found that more women than men feel sexual attrac­tion to both sexes (Lippa, 2006). Further, women who identify themselves as straight or lesbian may actually experience a greater range of sexual attraction and arousal than they are aware of. Laboratory research examined heterosexual and homosexual men’s and women’s physical and subjective arousal patterns by having the subjects watch movie clips of heterosexual, gay, and lesbian sexual encounters; a man mastur­bating; a woman masturbating; and bonobo apes mating. While watching each clip, subjects rated their subjective arousal on a keypad. Simultaneously, researchers mea­sured women participants’ physical arousal with a tampon-sized device that monitored increases in vaginal blood flow and resultant lubrication. Men wore an apparatus that fit on the penis and measured the degree of erection. The study found that women— regardless of their self-identified sexual orientation—experienced varying degrees of genital arousal in response to all of the video clips, including the mating bonobos. However, the women said that they were aroused only while viewing sexual activity that was compatible with their self-identified sexual orientation: Heterosexual women said that they were aroused only by heterosexual clips, and lesbians only by clips of women being sexual together or masturbating. In contrast, gay and straight men were physically aroused by the clips that they said they found arousing. Further, what turned men on was consistent with their sexual orientation. Gay men were aroused only by male-male sexual interaction and straight men by male-female and female-female material (Chivers et al., 2005).

Research with people who have high sex drives suggests further variability in the way sexual orientation expresses itself. Data from more than 3,600 research subjects showed that high sex drive in women who identified themselves as heterosexual was associated with increased sexual attraction to both men and women. The higher a woman’s sex drive, the more likely she was to feel sexual desire for both sexes. In contrast, high sex drive in straight men, gay men, and lesbians was associated with increased sexual attrac­tion only to one sex or the other. These findings are consistent across age groups and have been replicated in many regions of the world, including Latin America, Australia, India, and Western Europe (Lippa, 2006).