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Homosexual men tend to be later-born children. Slater’s index, the ratio of older sibs to all sibs, is consistently higher for male homosexuals than for comparable heterosexuals. According to some explanations of this tendency, homosexual men are later bom with respect to their brothers and later bom with respect to their sisters only secondarily and less strongly. We show that if sisters have no direct bearing ona brother’s sexual orientation and brothers do, then (older sisters/all sisters) = (older brothers + 1)/(all brothers + 2) On the other hand, if sisters have the same bearing on a brother’s sexual orientation as male sibs do, then (older sisters/all sisters) = (older brothers/all brothers) These ratios are calculated and compared in nine samples of homosexual men and nine corresponding samples of control heterosexuals. The first equation holds for homosexual men, and the second equation holds for heterosexual men. The late birth order of homosexual men is sex specific. What matters is a boy’s birth order relative to his brothers only. This effect may have its origins in an immune reaction or in behavioral contagion.