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Childhood gender nonconformity (femininity in males, masculinity in females) predicts a nonstraight (gay, lesbian, or bisexual) sexual orientation in adulthood. In previous work, nonstraight twins reported more childhood gender nonconformity than their genetically identical, but straight, cotwins. However, self-reports could be biased. We therefore assessed gender nonconformity via ratings of photographs from childhood and adulthood. These ratings came from independent observers naïve to study hypotheses. Identical twins with discordant sexual orientations (24 male pairs, 32 female pairs) visibly differed in their gender nonconformity from mid-childhood, with higher levels of gender nonconformity observed in the nonstraight twins. This difference was smaller than the analogous difference between identical twins who were concordant straight (4 male pairs, 11 female pairs) and identical twins unrelated to them who were concordant nonstraight (19 male pairs, 8 female pairs). Further, twins in discordant pairs correlated in their observer-rated gender nonconformity. Nongenetic factors likely differentiated the discordant twins’ gender-related characteristics in childhood, but shared influences made them similar in some respects. We further tested how recall of past rejection from others related to gender nonconformity. Rejection generally increased with gender nonconformity, but this effect varied by the twins’ sexual orientation.