Purpose: To explore the hypothesis that older siblings' smoking, after controlling for parents' smoking, influences children's smoking.
Methods: Study participants were 2981 students in the control cohort of a school-based smoking prevention randomized trial for whom parents' smoking and older siblings' smoking data were collected at 3rd grade and daily smoking data was assessed 9 years later through a self-report questionnaire at the 12th grade. Data were analyzed using conditional logistic regression and likelihood ratio tests.
Results: For families in which no parent smoked, the 12th grade prevalence of daily smoking was 31% when at least one older sibling smoked compared to 18% when no older sibling smoked. For families in which at least one parent smoked, the 12th grade prevalence of daily smoking was 41% when at least one older sibling smoked compared to 29% when no older sibling smoked. There was a substantial increase in the odds (OR = 1.60, p =.004) of children's daily smoking at 12th grade when their older siblings smoked, even after adjusting for parents' smoking.
Conclusions: Older siblings' smoking is associated with increased risk of children's smoking after adjusting for parents smoking. Furthermore, there was no evidence to suggest that the influence of older siblings' smoking was different in families where no parent smoked compared to those where at least one parent smoked. Also, there was no evidence that the influence of older siblings' smoking was different in boys vs. girls.