Genetic and environmental contributions to stability and change in children's internalizing and externalizing problems.

Objective: To estimate genetic, shared environmental, and nonshared environmental contributions to stability and change in internalizing and externalizing problems. Method: Maternal Child Behavior Checklist ratings were obtained for 3,873 twin pairs at age 3 and 1,924 twin pairs at age 7. For 1,575 twin pairs, ratings were available at both ages. Results: For Internalizing/Externalizing ratings, genetic, shared, and nonshared environmental factors explained about 59/51%, 10/30%, and 31/19% of the variance at age 3, and 40/52%, 31/32%, and 29/16% of the variance at age 7. The phenotypic correction of r = 0.38/0.54 between problems assessed at 3 and 7 years of age was explained for 66/55% by genetic factors, for 23/37% by shared environmental factors, and for 11/8% by nonshared environmental factors. The genetic, shared environmental, and nonshared environmental correlations between ages 3 and 7 were 0.51/0.57, 0.47/0.66, and 0.13/0.24, respectively. Conclusions: Genetic and shared environmental factors were most important for the stability of Internalizing and Externalizing Problems between ages 3 and 7. Nonshared environmental factors were mainly age-specific. For Internalizing Problems, shared environment may become more important from early to middle childhood