Association between co-twin sex and eating disorders in opposite sex twin pairs: evaluations in North American, Norwegian, and Swedish samples.

OBJECTIVE These three studies examined the hypothesis that prenatal exposure to sex hormones influences twins' risk for eating disorders based on co-twin sex, such that individuals with a female co-twin would be more likely than individuals with a male co-twin to meet diagnostic criteria for an eating disorder. METHODS Male and female twins from the United States (N=2607), Norway (N=2796) and Sweden (N=16,458) with known co-twin sex and zygosity were assessed for eating disorders. RESULTS In the U.S. and Swedish samples, sex was significantly associated with eating disorder diagnoses, and although co-twin sex was not associated with eating disorders overall, it was associated with broadly defined bulimia nervosa in the Swedish sample. The effects for bulimia were not sustained when monozygotic twins were excluded, suggesting that the effects of prenatal sex hormones play a minor role in influencing eating disorders. Sex and co-twin sex were not associated with eating disorders in the Norwegian sample. CONCLUSION The prenatal sex hormone hypothesis, which proposes that prenatal hormone exposure is associated with later eating disorder symptomatology, was not supported in these three population-based twin samples.