Sexuality. Human sexuality is the way people experience and express themselves sexually. This involves biological, psychological, physical, erotic, emotional, social, or spiritual feelings and behaviors. Because it is a broad term, which has varied with historical contexts over time, it lacks a precise definition. The biological and physical aspects of sexuality largely concern the human reproductive functions, including the human sexual response cycle. Someone's sexual orientation is their pattern of sexual interest in the opposite or same sex. Physical and emotional aspects of sexuality include bonds between individuals that are expressed through profound feelings or physical manifestations of love, trust, and care. Social aspects deal with the effects of human society on one's sexuality, while spirituality concerns an individual's spiritual connection with others. Sexuality also affects and is affected by cultural, political, legal, philosophical, moral, ethical, and religious aspects of life. Interest in sexual activity typically increases when an individual reaches puberty. Although no single theory on the cause of sexual orientation has yet gained widespread support, there is considerably more evidence supporting nonsocial causes of sexual orientation than social ones, especially for males. Hypothesized social causes are supported by only weak evidence, distorted by numerous confounding factors. This is further supported by cross-cultural evidence, because cultures that are tolerant of homosexuality do not have significantly higher rates of it. Evolutionary perspectives on human coupling, reproduction and reproduction strategies, and social learning theory provide further views of sexuality. Sociocultural aspects of sexuality include historical developments and religious beliefs. Some cultures have been described as sexually repressive. The study of sexuality also includes human identity within social groups, sexually transmitted infections, and birth control methods. There is considerably more evidence supporting innate causes of sexual orientation than learned 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 differences or 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 people 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 genes causing homosexuality 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 for this hypothesis. See also: Sex differences in psychology § Sexual behavior Psychological theories exist regarding the development and expression of gender differences in human sexuality. A number of them agree in predicting that men should be more approving of casual sex and should also be more promiscuous than women.