Genetic and Environmental Influences on the Development of Irritability from Ages 3 to 5 Years.

OBJECTIVE Irritability is genetically influenced and is associated with internalizing and externalizing psychopathology. However, little is known about the etiology of the development of irritability in the preschool period. The present study examined this from rank-order stability and developmental trajectories perspectives. METHOD The irritability of 310 same-sex twin pairs (monozygotic = 123; dizygotic = 187; 51% female) was longitudinally assessed at ages 3, 4 and 5 years via parent-reports on the Child Behavior Checklist. Biometric Cholesky models and latent growth curve models were used to examine genetic and environmental influences on both the rank-order stability and instability, and within individual changes in levels of irritability across age (i.e. developmental trajectories), respectively. RESULTS The heritability of irritability ranged from 53% to 60%, with the remaining variances explained by nonshared environmental influences. Age-to-age stability was largely due to genetic influences. Novel genetic and nonshared environmental effects emerged at ages 4 and 5, indicating genetic and environmental contributions to instability. Approximately 42% and 22% of genetic influences at ages 4 and 5, respectively, were independent of prior ages, and over 85% of nonshared environmental influences were age-specific. Individual differences in developmental trajectories of irritability were entirely due to nonshared environmental influences. CONCLUSIONS The rank-order stability of irritability and within-individual change in levels of irritability across age are governed by different etiological processes, emphasizing the importance of examining development from multiple perspectives. Both perspectives highlight the role of nonshared environmental factors in early irritability development. Interventions could benefit from leveraging these factors to redirect early irritability development.