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A correlation between fraternal birth order and male sexual orientation has been suggested by research. Ray Blanchard identified the association and referred to it as the fraternal birth order effect. In several studies, the observation is that the more older brothers a man has from the same mother, the greater the probability is that he will have a homosexual orientation. It has sometimes been called the ''older brother effect''. It has been estimated that 15% of the homosexual demographic is associated with fraternal birth order.〔Valenzuela C. 2009. "Sexual Orientation, Handedness, Sex Ratio, and Fetomaternal Tolerance-Rejection". ''Biological Research'' 43(3), 347–356〕 ==Empirical basis== The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men." According to several studies, each older brother increases a man's odds of having a homosexual orientation by 28–48%. The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters.〔 The fraternal birth order effect has also been observed among male-to-female transsexuals: MtF transsexuals who are sexually interested in men have a greater number of older brothers than MtF transsexuals who are sexually interested in women. This has been reported in samples from Canada, the United Kingdom, the Netherlands, and Polynesia. The effect has also been observed amongst paedophiles. In one survey, gay and bisexual male paedophiles were typically younger than heterosexual male paedophiles, primarily the result of the homosexual/bisexual group being born later among their brothers, generally speaking.〔Bogaert, Anthony F., et al. ("Pedophilia, sexual orientation, and birth order." ) ''Journal of Abnormal Psychology'' 106.2 (1997): 331.〕 The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation.〔 To explain this finding, a maternal immune response has been hypothesized. Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates.〔 Other studies have suggested the influence of birth order was not due to a biological, but a social process.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fraternal birth order and male sexual orientation」の詳細全文を読む スポンサード リンク
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