The Growing Up in Ireland study allow us to examine whether smoking conveys an increased risk for behavioral problems when we take account of these social factors. Importantly, the study collected information on the mother’s level of smoking in pregnancy and this provides us with an important additional tool with which to corroborate the causal relationship between exposure to cigarette smoke in the womb and behavioral problems at age 9. If the strength of the relationship between smoking and behavioral problems increases with the level of maternal smoking, this is more persuasive than a simple association.

In a paper recently published in the Journal of Abnormal Child Psychology, Cathal McCrory and Richard Layte (McCrory & Layte 2012) did just this. They showed that the risk of the child being reported by his/her teacher as having conduct, attention or hyperactivity problems at age 9 was significantly related to whether the mother smoked during pregnancy and, moreover, that the risk increased with the number of cigarettes smoked during pregnancy.