In this article, the authors investigated the effect of maternal smoking during pregnancy on behavioral problems, which were not mediated by lower birth weight, in offspring at 3 y of age. The authors used the Child Behavior Checklist for ages 2-3 y (CBCL/2-3; Achenbach, Edelbrock and Howell) to assess behavioral problems in 1,377 2- to 3-y-old healthy twin pairs. Soon after the birth of twins,...