A longitudinal analysis of anger and inhibitory control in twins from 12 to 36 months of age.

Inhibitory control (IC) is a dimension of child temperament that involves the self-regulation of behavioral responses under some form of instruction or expectation. Although IC is posited to appear in toddlerhood, the voluntary control of emotions such as anger begins earlier. Little research has analyzed relations between emotional development in infancy and later emerging IC. We examined phenotypic associations and genetic and environmental influences on parent- and laboratory-assessed anger and IC in a twin sample from 12 to 36 months of age. Typically, twins with low levels of IC had high levels of anger. Behavioral genetic findings confirmed significant genetic influences on anger and IC as assessed by parents, and on lab-based anger assessments. Shared environmental factors contributed to twin similarity on lab-assessed anger and IC at 36 months. Phenotypic covariance between anger and IC was largely due to overlapping genetic factors for parent ratings, and environmental factors in the laboratory.