Effects of Birth Order on Human Sexuality: the Sex of Siblings Matters

Birth order is known to influence various personality dimensions, from intelligence to personality traits; however, its relationship with human sexuality remains substantially underexplored. This study aimed to investigate the relationship between birth order and aspects of human sexuality, specifically: onset of sexual activity, sociosexual orientation, lifetime number of sexual partners, and sexual dominance. Data on birth order and sexuality were procured through a large-scale online questionnaire circulated among the Czech population, with the final analysis sample (N = 4,495) comprising respondents from two-child families. Significant differences were observed between firstborn and secondborn individuals. Notably, secondborns initiated their sexual activity earlier, while associations between birth order and number of sexual partners were contingent on the sex of the respondents and their siblings. No association between birth order and sociosexual score (measured with the SOI-R) was observed in women, and only a weak association was found in men. Both dominant and submissive sexual preferences correlated with higher birth order in both sexes. Our findings indicate that firstborn and secondborn individuals exhibit differences in sexual desires, behaviors, attitudes, and preferences. Future research exploring the relationship between birth order and sexual behavior/preferences should account for sibling sex as a pivotal factor.