Influence of genes and family environment on adult smoking behavior assessed in an adoption study

Twin studies suggest that genetic factors influence smoking behavior. However, in these studies, genetic and environmental influences may be confounded. We examined whether smoking behavior of adoptees is associated with smoking behavior in adoptive and biological relatives in a design in which this confounding is minimized. Data on smoking status were collected using a mailed questionnaire in a sample of 840 adoptive families from the Danish Adoption Register including all non-familial adoptions from 1924 through 1947. Smoking data were available for 706 adoptees, 451 biological fathers, 580 biological mothers, 641 adoptive fathers, 693 adoptive mothers, 199 biological full-siblings, 684 maternal half-siblings, and 576 paternal half-siblings. Adoptees' status as ever, current, heavy, or former (ex-)smokers was related to their relatives' smoking status. Adoptees ever smoking was associated with their full-siblings' status as ever smokers (OR = 3.6 [1.5-8.7]). Adoptees' current smoking was also associated with their full-siblings' ever (OR = 3.2 [1.5-7.7]) and current (OR = 2.6 [1.3-5.0]) smoking, whereas adoptees' heavy smoking was associated with their full-siblings ever (OR = 5.5 [2.1-14.3]), current (OR = 4.0[1.9-8.6]) and heavy (OR = 2.0[1.0-4.2]) smoking. Compared with current smokers, adoptee ex-smokers had full-siblings who were ex-smokers more often (OR = 3.5 [1.0-11.6]), current (OR = 0.4 [0.2-0.8]), and heavy (OR = 0.3 [0.1-0.9]) smokers less often. Adoptees' smoking behavior was not associated with adoptive or biological parents' status as current smokers. This study of smoking behavior in adult adoptees and their biological and adoptee family supports the finding in twin studies of a genetic influence on smoking within the same generation. Genet. Epidemiol. 21:193-200, 2001. (C) 2001 Wiley-Liss, Inc.