Critics of adoption studies maintain that because adoptive parents are better educated and have greater economic resources than the general population, environmental effects measured in adoptive families are greatly attenuated36. Indeed, the socioeconomic status of adoptive families in our sample was significantly higher than that of the non-adoptive families. However, statistically adjusting for family SES did not eliminate the larger and more diverse effects of parent smoking in non-adoptive families. Of course, the larger effects in non-adoptive families may be due to factors independent of SES that are differentially present in the environments of adoptive and non-adoptive families. In addition, the increase in disruptive behavior disorder symptoms in the adoptees could suggest that they represent a sample at high genetic risk where the effect of exposure to parental smoking might be diminished. Although we cannot rule this out, we believe it is an unlikely explanation for our findings. In fact, we and others25, 37 have shown that the majority of adoptees are psychologically well-adjusted with only a modest, albeit significant, increase in risk for symptoms of disruptive disorders.
