Normative development of physical aggression from 8 to 26 months

Abstract

This study investigated the normative use and developmental course of physical aggression (PA), defined as use of physical force such as hitting, biting, and kicking, from 8 to 26 months and predictors thereof. We used data from the Behavior Outlook Norwegian Developmental Study, comprising 1,159 children (559 girls and 600 boys). Both mothers and fathers reported frequently about their child's use of PA in personal and telephone interviews. Mean number of reports per child was 7.16 (SD 1.70), with 90% having at least 6 reports. We employed Rasch scaling to construct a single linear PA measure and multilevel growth curve modeling to address the research questions. The results confirm that the development of PA is nonlinear, with a peak in frequency at about 20-22 months, which is followed by a decline toward 26 months. There is both within- and between-child variance in the development of PA. Higher levels of PA were predicted by the presence of a same-age sibling, maternal and paternal mental distress, and difficult child temperament (high activity level and distress due to limitations), whereas the main effect of gender was only trend-significant. Growth of PA across this developmental period was predicted by the presence of a same-age sibling and high activity level. The results both support and transcend previous research within this field.