initiated drinking at age 14 than age and cohort matched twins from same-sex pairs (Rose et al. 2001). Taken together, this literature suggests that aspects of the sibling relationship (e.g. sibling interdependence, opposite-sex sibling pairs) have effects on adolescent drinking. However, no study to our knowledge has examined whether sex of the co-twin has an independent effect on adolescent drinking. Therefore, we examined whether sex of the co-twin modified the influence on genetic risk on alcohol outcomes across development. Results indicated that the association between genetic risk (both AUD-GR and EXT-GR) and intoxication frequency was greater among 14 and 17 year old females with male co-twins. This effect was not observed in young adulthood, likely due to the decrease in interdependence and the increase of unique environmental influences that the twins begin to encounter at that age (Penninkilampi-Kerola et al., 2005).
