This study investigated joint effects of maternal prenatal smoking and parental history of antisocial behavior on physical aggression between ages 17 and 42 months in a population sample of children born in Québec (N = 1,745). An analysis of variance (ANOVA) showed significant main effects of maternal prenatal smoking and a significant interaction between maternal prenatal smoking and mother's history of antisocial behavior in the prediction of children's probability to display high and rising physical aggression. The interaction indicated that the effects of heavy smoking during pregnancy (≥10 cigarettes/day) were greater when the mother also had a serious history of antisocial behavior. The effects remained significant after the introduction of control variables (e.g., hostile-reactive parenting, family functioning, parental separation/divorce, family income, and maternal education). Another significant interaction not accounted for by control variables was observed for maternal prenatal smoking and family income, indicating more serious effects of maternal prenatal smoking under relatively low-income, conditions. Both interactions indicate critical adversities that, in combination with maternal prenatal smoking, have supraadditive effects on (the development of) physical aggression during early childhood. These findings may have implications for the selection of intervention targets and strategies.Physical aggression is present in the majority of young children and it decreases with age as emotion regulation skills and alternative conflict-resolution strategies emerge, aided by development of language and cognitive abilities (Tremblay, 2000;Tremblay & Nagin, 2005). However, it is also clear that those children who show the highest levels of physical aggression during early childhood are most at risk of chronic physical aggression throughout childhood (Côté, Vaillancourt, LeBlanc, Nagin, & Tremblay, 2006; NICHD Early Child Care Research Network, 2004), followed by serious violent and nonviolent delinquency during adolescence (Broidy et al., 2003;Nagin & Tremblay, 1999). Recent studies show that children with early, persistent aggression show not only more externalizing problems at
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CIHR Author Manuscriptfollow-up but also more internalizing and peer problems (e.g., depression, loneliness, poor friendship quality) compared with children on the lower and desisting aggression trajectories (Campbell, Spieker, Burchinal, Poe, & The NICHD Early Child Care Research Network, 2006). It is therefore important to determine which factors or combinations of factors predict consistently high physical aggression during early childhood.Research has linked maternal prenatal smoking to early childhood physical aggression (Huijbregts, Séguin, Zoccolillo, Boivin, & Tremblay, 2007;Tremblay et al., 2004) and to other externalizing (Orlebeke, Knol, & Verhulst, 1997) and disruptive behavior problems (Wakschlag, Leventhal, Pine, Pickett, & Carter, 2006). Maternal prenatal smoking...