Influences of genes and shared family environment on adult body mass index assessed in an adoption study by a comprehensive path model

The aim of this work was to explore the influences of the shared familial environment and the nonshared individual environment on body mass index in light of convincing evidence for genetic influence. A cross-sectional adoption design was used, including information on adult adoptees and their biological fathers and mothers, biological full siblings and paternal and maternal half siblings, and adoptive fathers and mothers. Body mass index (weight/height²) was derived from reported height and weight. A model of familial resemblance based on path analysis is used to test for effects of genetic influence and for effects of the environment shared among family members: transmission from the adoptive father and adoptive mother to the adoptee, from biological father and biological mother to biological children they reared, from the biological father to maternal half-siblings, from the biological mother to paternal half-siblings, and among biological siblings who may have lived together. The model also incorporates effects of assortive mating of the biological parents and of the adoptive parents, of shared preadoptive environmental influences between the biological mother and the adoptee, and of selective placement of the adoptee. The estimated heritability was 0.34 (standard error 0.03). No parameter indicating effects of shared familial environment either before or after adoption, assortative mating, or selective placement was significant. There was strong evidence for genetic effects, and no evidence for any effects related to the shared family environment - all familial resemblance in adults can be attributed to genetic influences. However, more than half of the interindividual differences in body mass index is due to nonshared individual environmental influences.