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Self-identified bisexual men report high sexual arousal to both male and female stimuli, but no study to date has compellingly demonstrated that such men have a bisexual pattern of genital arousal. We examined sexual arousal patterns among bisexual men recruited using stringent criteria designed to exclude those who were less likely to have sexual interest in both sexes. Furthermore, we included a bisexual stimulus depicting a man engaged in sex simultaneously with another man and a woman. On average, the bisexual men showed a bisexual arousal pattern, with respect to both self-reported and genital arousal. Additionally, the bisexual men were more aroused by the bisexual stimulus compared with the homosexual and heterosexual men. Some bisexual-identified men have bisexual genital arousal patterns, although it remains unclear how common they are.

Studies of subjective and genital sexual arousal in monosexual (i.e. heterosexual and homosexual) men have repeatedly found that erotic stimuli depicting men's preferred sex produce strong responses, whereas erotic stimuli depicting the other sex produce much weaker responses. Inconsistent results have previously been obtained in bisexual men, who have sometimes demonstrated distinctly bisexual responses, but other times demonstrated patterns more similar to those observed in monosexual men. We used fMRI to investigate neural correlates of responses to erotic pictures and videos in heterosexual, bisexual, and homosexual men, ages 25-50. Sixty participants were included in video analyses, and 62 were included in picture analyses. We focused on the ventral striatum (VS), due to its association with incentive motivation. Patterns were consistent with sexual orientation, with heterosexual and homosexual men showing female-favoring and male-favoring responses, respectively. Bisexual men tended to show less differentiation between male and female stimuli. Consistent patterns were observed in the whole brain, including the VS, and also in additional regions such as occipitotemporal, anterior cingulate, and orbitofrontal cortices. This study extends previous findings of gender-specific neural responses in monosexual men, and provides initial evidence for distinct brain activity patterns in bisexual men. Sexual Orientation and Sexual MotivationThe term sexual orientation is used to indicate an individual's sexual arousal and attraction patterns, sexual behavior patterns, or sexual identity 1 , which often go together but can also differ from each other [2][3][4][5][6] . While most individuals identify as heterosexual, a significant number of individuals also report identifying as homosexual (1.9-2% of the US population) or bisexual (2-4% of the US population), with even greater proportions reporting some degree of same-sex behavior or attraction 7,8 .If the term sexual orientation is used to describe a pattern of arousal and attraction 1 , then genital assessment has high face validity for studying sexual orientation in men. Neuroimaging, however, may have a variety of methodological advantages, including the potential for greater sensitivity in detecting motivational responses to stimuli that are psychologically significant yet unlikely to result in noticeable physiological changes 9 or even subjective responses 10 . Even with briefly-presented erotic pictures, fMRI has demonstrated a high degree of sensitivity and specificity in measuring sexual orientation [11][12][13] .Furthermore, genital arousal is capable only of indicating degree of increase or decrease along a single dimension of tumescence, and thus provides little qualitative information on the mental states underlying sexual arousal and desire. Neuroimaging techniques, in contrast, can localize activity within various brain structures, and therefore suggest and test hypotheses about diverse psychological processes influencing sexuality. The present research f...