The proposed study aims to understand the relationship between prenatal smoke exposure and aggression in children. The study will expand on previous findings by also investigating the extent to which this relationship may be mediated by genetic factors; a mother's smoking behavior during pregnancy may be influenced by her own predisposition toward antisocial behavior. Aggression, along with other traits such as cognitive ability in 9 year old twins will be measured using in depth interviews and multiple informant behavior assessments. The mothers will be assessed for antisocial behavior in similar manner, including ascertainment of current and prenatal substance abuse. This study will be conducted in a genetically informative twin design, which will allow for the investigation of the smoking/aggression relationship while controlling for covariates. It is hypothesized that children exposed in utero to high levels of cigarette smoke will exhibit elevated aggression. Also hypothesized, however, is that the said relationship will be at least partially explained by other risk factors, such as genetic predisposition towards anti-social behavior in the mother. Birth complications will also be examined for possible exacerbation of the smoking/aggression relationship as evidenced in prior studies. The effects of smoke exposure are expected to be greatest in the presence of birth complications.