a1 Research Unit on Children's Psychosocial Adjustment, Ste-Justine Hospital Research Center, University of Montreal, QC, Canada
a2 Department of Sociology, University of Montreal, QC, Canada
a3 School of Psychology, Laval University, QC, Canada
a4 Department of Psychology, Université du Québec à Montréal, QC, Canada
a5 National Center for Children and Families, Teachers College, Columbia University, NY, USA
a6 Department of Psychoeducation, University of Montreal, QC, Canada
a7 School of Criminology, University of Montreal, and Mental Health Institute of Montreal Research Center, QC, Canada
a8 School of Public Health, Physiotherapy and Population Science, University College Dublin, Republic of Ireland
a9 Department of Psychology and Pediatrics, University of Montreal, QC, Canada
Background Physical aggression (PA) tends to have its onset in infancy and to increase rapidly in frequency. Very little is known about the genetic and environmental etiology of PA development during early childhood. We investigated the temporal pattern of genetic and environmental etiology of PA during this crucial developmental period.
Method Participants were 667 twin pairs, including 254 monozygotic and 413 dizygotic pairs, from the ongoing longitudinal Quebec Newborn Twin Study. Maternal reports of PA were obtained from three waves of data at 20, 32 and 50 months. These reports were analysed using a biometric Cholesky decomposition and linear latent growth curve model.
Results The best-fitting Cholesky model revealed developmentally dynamic effects, mostly genetic attenuation and innovation. The contribution of genetic factors at 20 months substantially decreased over time, while new genetic effects appeared later on. The linear latent growth curve model revealed a significant moderate increase in PA from 20 to 50 months. Two separate sets of uncorrelated genetic factors accounted for the variation in initial level and growth rate. Non-shared and shared environments had no effect on the stability, initial status and growth rate in PA.
Conclusions Genetic factors underlie PA frequency and stability during early childhood; they are also responsible for initial status and growth rate in PA. The contribution of shared environment is modest, and perhaps limited, as it appears only at 50 months. Future research should investigate the complex nature of these dynamic genetic factors through genetic–environment correlation (r GE) and interaction (G × E) analyses.
(Received November 16 2012)
(Revised November 26 2013)
(Accepted December 07 2013)
c1 Address for correspondence: E. Lacourse, Ph.D., Department of Sociology, CP 6128, succursale Centre-ville, Montreal, Quebec, H3C 3J7, Canada. (Email: email@example.com) [E.L.] (Email: firstname.lastname@example.org) [M.B.]