The association between body mass index and body bulk with externalizing behavior: a longitudinal twin study.

INTRODUCTION: The current study examined longitudinal links between adolescent body mass index and body bulk with externalizing behaviors using data from the University of Southern California's Risk Factors for Antisocial Behavior twin project. METHODS: The sample included 1,260 individuals (51% females), with 603 twin pairs (254 monozygotic; 349 dizygotic). Body mass index and body bulk were derived from measures of height and weight obtained at age 13-14. Externalizing behavior was measured using the Youth Self-Report at age 13-14 and the Adult Self-Report at age 19-20. RESULTS: Higher body mass index and greater body bulk were associated with higher levels of externalizing behavior both concurrently at age 13-14, and prospectively at age 19-20 (B's = 0.11-0.42, p's < .05). Twin modeling indicated high heritability for body mass index and body bulk (h2 = 0.73-0.81) and moderate heritability for externalizing behavior (h2 = 0.31-0.41). Bivariate models suggested that genetic factors contributed to the modest covariance between body size and externalizing behavior, although the genetic correlation estimates were imprecise. CONCLUSION: Findings are consistent with the possibility that some genetic influences are shared between body size and externalizing behavior; however, the modest phenotypic associations and uncertainty surrounding the genetic correlation estimates warrant cautious interpretation.