Behavioral genetic research on callous-unemotional behaviors (CU) in children shows that CU is genetically influenced as early as 2 years of age, and that there are genetic contributions to stability and change in toddlerhood and in middle to older childhood (M. Flom & K. J. Saudino, 2016, Dev Psychopathol, 29, 1227-1234; N. M. G. Fontaine et al., 2010, J Am Acad Child Adolesc Psychiatry, 49, 656-664; E. Viding & E. J. McCrory, 2012, Dev Psychopathol, 24, 969-983). Nothing, however, is known about the underlying etiology of development across the preschool years, when children undergo vast changes in moral and socio-emotional development. The present study expands upon previous work by using a new Boston University Twin Project cohort of 300 twin pairs at ages 3, 4, and 5 years (MZ=123; DZ=187). CU was assessed using the parent-reported Child Behavior Checklist 1.5-5 (T. M. Achenbach & L. A. Rescorla, 2000), and demonstrated moderate stability across age (r ranging from .44 to .52). Consistent with previous work at different ages, model-fitting analyses revealed little differential heritability across age (age 3 h2=.64 [.55-.72]; age 4=.64 [.54-.71]; age 5=.58 [.46-.67]), with nonshared environmental influences explaining remaining variance. Genetic correlations between age 3 and later ages were substantial (age 3 and age 4 rg=.74 [.65-.82]; age 3 and age 5 rg=.72 [.62-.82]), and continuity across these intervals was due solely to genetic effects. In contrast, the genetic correlation between CU at ages 4 and 5 was lower, and there were both genetic and nonshared environmental contributions to stability (rg=.54 [.44-.64]; re=.43 [.30-.53]). Although there is substantial genetic covariation from 3 years on, there was also considerable novel genetic variance coming online. Age-specific genetic effects accounted for roughly 70% of the genetic variance in CU at ages 4 and 5. Nonshared environmental effects were primarily age-specific, though modest continuity across ages 4 and 5 explained 10% of the overall nonshared environmental effects at age 5. As with research in toddlers and older children, these results confirm that CU is genetically influenced in preschoolers, with genetic factors contributing to both change and stability and nonshared environmental factors primarily influencing change. The minimal, but significant, contribution of the nonshared environment to stability between CU at ages 4 and 5 is different from past research, and may represent small carry-over effects of unique peer or teacher influences that begin emerging at age 4, as more children in our sample enter preschool.
