self-report or physiologically. Multiple physiological methods exist, including measurement of penile erection, viewing time, FMRI, and pupil dilation. In men Human male sexuality encompasses a wide variety of feelings and behaviors. Men's feelings of attraction may be caused by various physical and social traits of their potential partner. Men's sexual behavior can be affected by many factors, including evolved predispositions, individual personality, upbringing, and culture. While most men are heterosexual, and attracted only to women, there are signi S… Human male sexuality encompasses a wide variety of feelings and behaviors. Men's feelings of attraction may be caused by various physical and social traits of their potential partner. Men's sexual behavior can be affected by many factors, including evolved predispositions, individual personality, upbringing, and culture. While most men are heterosexual, and attracted only to women, there are significant minorities of homosexual men and varying degrees of bisexual men. Homosexual males are more common than homosexual females. Women with a relatively low waist-to-hip ratio (WHR) are considered more attractive. The exact ratio varies among cultures, depending on the WHR of the women in the local culture. In Western cultures, a WHR of 0.70 is preferred. Other physical factors of attraction include full lips, facial femininity, long, lustrous hair, low body mass index, and low waist circumference. Preference for a slim or a plump body build is culturally variable, but in a predictable manner. In cultures where food is scarce, plumpness is associated with higher status and is more attractive, but the reverse is true in wealthy cultures. Men generally prefer their wives to be younger than they are, but by how much exactly varies between cultures. Older men prefer greater age differences, while teenage males prefer females slightly older than they are. The exact degree to which physical appearance is considered important in selecting a long-term mate varies between cultures. Sexual orientation refers to one's relative attraction to men, to women, or to both. Most researchers studying sexual orientation focus on patterns of attraction rather than behavior or identity, because culture affects the expression of behavior or identity and it is attraction that motivates behavior and identity, not the other way around. Aside from being heterosexual or homosexual, individuals can be any of varying degrees of bisexual. Bailey et al. stated that they expect that in all cultures the vast majority of people are sexually predisposed exclusively to the other sex, with a minority being sexually predisposed to the same sex, whether exclusively or not. In Western surveys, about 93% of men identify as completely heterosexual, 4% as mostly heterosexual, 0.5% as more evenly bisexual, 0.5% as mostly homosexual, and 2% as completely homosexual. An analysis of 67 studies found that the lifetime prevalence of sex between men (regardless of orientation) was 3–5% for East Asia, 6–12% for South and South East Asia, 6–15% for Eastern Europe, and 6–20% for Latin America. The World Health Organization estimates a worldwide prevalence of men who have sex with men between 3 and 16%. Sexual orientation can be measured via self-report or physiologically. Multiple physiological methods exist, including measurement of penile erection, viewing time, FMRI, and pupil dilation. In men, these all show a high degree of correlation with self-report measures, including men who self report as "mostly straight" or "mostly gay". What impact same-sex sexuality has upon one's social identity varies across cultures. The question of precisely how cultures through history conceptualized homosexual desire and behavior is a matter of some debate. In much of the modern world, sexual identity is defined based on the sex of one's partner. In some parts of the world, however, sexuality is often socially defined based on sexual roles, whether one is a penetrator ("top") or is penetrated ("bottom"). Although no causal theory has yet gained widespread support, there is considerably more evidence supporting nonsocial causes of sexual orientation than social ones, especially for males. This evidence includes the cross-cultural correlation of homosexuality and childhood gender nonconformity, moderate genetic influences found in twin studies, evidence for prenatal hormonal effects on brain organization, the fraternal birth order effect, and the finding that in rare cases where infant males were raised as girls due to physical deformity, they nevertheless turned out attracted to females. Hypothesized social causes are supported by only weak evidence, distorted by numerous confounding factors. Cross-cultural evidence also leans more toward non-social causes. Cultures that are very tolerant of homosexuality do not have significantly higher rates of it. Homosexual behavior is relatively common among boys in British single-sex boarding schools, but adult Britons who attended such schools are no more likely to engage in homosexual behavior than those who did not. In an extreme case, the Sambia ritually require their boys to engage in homosexual behavior during adolescence before they have any access to females, yet most of these boys become heterosexual. It is not fully understood why the genes for homosexuality, or allowing it to develop, whatever they may be, persist in the gene pool. One hypothesis involves kin selection, suggesting that homosexuals invest heavily enough in their relatives to offset the cost of not reproducing as much directly. This has not been supported by studies in Western cultures, but several studies in Samoa have found some support for this hypothesis. Another hypothesis involves sexually antagonistic genes, which cause homosexuality when expressed in males but increase reproduction when expressed in females. Studies in both Western and non-Western cultures have found support