Effects of sibship size and composition on younger brothers' and sisters' alcohol use initiation: findings from an Australian twin sample.
BACKGROUND
The effects of sibship size and structure on delinquency are well established. Specifically, having a large family and many brothers has been shown to predict offending. However, despite strong links between delinquency and alcohol use, the contribution of sibship factors to drinking behaviors remains largely unexplored. The current study investigated the impact of sibship size and composition on younger brothers' and sisters' ages of drinking and intoxication onset.


METHODS
We employed a sample of 4,281 same-sex twins from the Australian Twin Register to examine whether (i) large sibship size facilitates earlier age at first drink (AFD) and age at first intoxication (AFI) in males and females, (ii) having many older brothers predicts earlier ages of AFD and AFI in males, and (iii) having many older brothers results in later AFD and AFI in females. We tested whether effects were moderated by parental divorce and alcohol misuse and mediated by familial religion.


RESULTS
Sibling effects were minimal before accounting for family context. However, when parental divorce and excessive parental drinking were included as moderators, sibling effects were significantly amplified among individuals from homes of divorce, and effects were strongest when siblings were close in age.


CONCLUSIONS
Strong close in age older sibling effects indicate that proximal sibling attitudes and behaviors about alcohol likely interact with structural factors to influence younger siblings' drinking. Sibship factors were much more influential in one population (individuals from homes of divorce) than another (respondents with a parental history of excessive drinking), suggesting that sibling effects vary depending on the type of co-occurring familial risk. Prevention efforts performed at the family level, and introduced before first use of alcohol, are likely to delay drinking initiation and help prevent future alcohol problems.