Although genetic and environmental influences on behavior problems in middle childhood and adolescence have been well-studied, little is known about the etiology of behavior problems in very early childhood. The present study explores genetic and environmental contributions to individual differences in behavior problems and competences in an infant-toddler sample of twins. There were 1,950 twin pairs (mean age = 23.8 months) who were rated by parents on the InfantToddler Social and Emotional Assessment. All four domains (Externalizing, Internalizing, Dysregulation, Competence) and 20 subscales-indices on the Infant-Toddler Social and Emotional Assessment displayed significant heritability. There were also substantial shared environmental influences operating on most of the domains and subscales. Compared with behavior problems, behavioral competencies were less heritable and more influenced by shared environments.
Keywords twins; behavior problems; genetics; model fittingThe toddler period may be a critical time for the early emergence of clinically significant problem behaviors (Campbell, 1985;Keenan & Wakschlag, 2000). Externalizing problems (e.g., aggression, conduct disorders, hyperactivity) and internalizing problems (e.g., anxiety, depression, and emotional problems) can be reliably identified in very early childhood and show considerable stability across age (e.g., Briggs-Gowan et al., 2006;Campbell, Pierce, March, Ewing, & Szumowski, 1994; Fischer, Rolf, Haszi, & Cummings, 1984;Keenan, Shaw, Delliquadri, Giovannelli, & Walsh, 1998;Lavigne et al., 1998;Mathiesen & Sanson, 2000;Olson, Bates, Sandy, & Lanthier, 2000). Toddlers who display problem behaviors are at risk for a variety of nonoptimal developmental outcomes including poor academic performance, conflictual parent-child interactions, peer rejection, delinquency, and later Copyright 2008 by the American Psychological Association Correspondence concerning this article should be addressed to Kimberly J. Saudino, Department of Psychology, Boston University, 64 Cummington Street, Boston, MA 02215. ksaudino@bu.edu. maladjustment (e.g., Caspi, Moffitt, Newman, & Silva, 1996;Hofstra, van der Ende, & Verhulst, 2002;Patterson, DeBaryshe, & Ramsey, 1989;Stevenson & Goodman, 2001). Given their developmental significance, it is important to understand the factors that influence problem behaviors in very young children.
NIH Public AccessThe etiology of behavior problems in middle childhood and adolescence has been well studied. Twin and adoption studies examining externalizing, internalizing, and related behavior problems consistently reveal substantial genetic influence, with heritabilities in the range of .40-.70 (e.g., Bartels et al., 2003Bartels et al., , 2004Edelbrock, Rende, Plomin, & Thompson, 1995;Leve, Winebarger, Fagot, Reid, & Goldsmith, 1998;Saudino, Ronald, & Plomin, 2005). The influence of shared environments on behavior problems is typically more modest, explaining between 0% and 40% of the variance in behavior problems (e.g., Bartels et...