We then examined whether drinking across development differed by co-twin sex. Among same sex twins, drinking outcomes did not differ by sex at ages 12 (β = 0.005, p=0.874), 14 (β = 0.006, p=0.794), 17 (β = 0.041, p=0.087), or 22 (β = 0.005, p=0.919). Among opposite sex twins, drinking outcomes differed by co-twin sex at ages 12 (β = 0.124, p<0.048) and 14 (β = 0.133, p<0.001), in that females with male co-twins were more likely to have initiated alcohol use (age 12) and drink more frequently (age 14) than males with female co-twins.
