Twin pairs in which at least one twin is homosexual were solicited through announcements in the gay press and personal referrals from to the present. Thirty-eight pairs of monozygotic twins 34 male pairs and 4 female pairs were found to have a concordance rate of Twenty-three pairs of dizygotic twins were found to have a concordance rate of In addition, three sets of triplets were obtained. Two sets contained a pair of monozygotic twins concordant for sexual orientation with the third triplet dizygotic and discordant for homosexual orientation. A third triplet set was monozygotic with all three concordant for homosexual orientation.
Biology and sexual orientation
Sexual Arousal Patterns of Identical Twins with Discordant Sexual Orientations | Scientific Reports
A new study of twins provides the strongest evidence yet that homosexuality has a genetic basis, researchers say, though they say other factors like social conditioning may be important. The study, published in the December issue of The Archives of General Psychiatry, adds to evidence that sexual orientation does not result from a maladjustment or moral defect, one author said. Michael Bailey, an assistant professor of psychology at Northwestern University in Evanston, "which is exactly the kind of pattern you would want to see if something genetic were going on. Bailey was referring to brothers by adoption. The study examined 56 identical twins, 54 fraternal twins and 57 adoptive brothers recruited through advertisements in gay-interest publications. Identical twins are genetic clones, having developed in the womb from a single egg that split after being fertilized by a single sperm.
Sexual Arousal Patterns of Identical Twins with Discordant Sexual Orientations
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A growing amount of research suggests that sexual orientation has a genetic basis. In fact, scientists recently identified two specific genes that appear to differ between gay and straight men . If sexual orientation is indeed genetically determined, it would be tempting to assume that identical twins would always have the same orientation, right? If they have the exact same genes and our genes control our sexuality, this would seem like a pretty logical conclusion. Identical twins sometimes have different orientations.