Impulsivity-hyperactivity and subtypes of aggression in early childhood: an observational and short-term longitudinal study

Although not a central goal of the study, two concurrent regression models were conducted to test if impulsivity-hyperactivity at time 1 was uniquely associated with relational and physical aggression at time 1. Impulsivity-hyperactivity (β = 0.25) accounted for a significant amount of unique variance in the association with relational aggression, controlling for physical aggression and gender, ΔF(1, 101) = 5.94, P = 0.017, ΔR ² = 0.06). In the next model, impulsivity-hyperactivity (β = 0.20) tended to account for a unique (but not significant) amount of variance in the association with physical aggression, controlling for relational aggression and gender, ΔF(1, 101) = 3.61, P = 0.06, ΔR ² = 0.03).

A socioeconomic status (SES) index was created based on available information on household income and highest parental education level for each parent/adult living in the home. SES was entered at step 1 in the regression models as a covariate and the findings were virtually identical. Thus, for ease of communication these findings are not reported.

In order to extend the relevancy of the current findings to clinical practice, we also examined our findings using a categorical approach. A small percentage of children were high on both initial aggression and impulsivity-hyperactivity (7.2% overall; 2.7% of physically aggressive children, 2.7% of relationally aggressive children, and 1.8% of the both physically and relationally aggressive group). Although the cell sizes were very small, given the potential clinical interest in these categorical comparisons we thus conducted a 2 (focal child gender) × 4 (co-morbid status: nonaggressive and nonimpulsive–nonhyperactive; physically aggressive and impulsive-hyperactive; relationally aggressive and impulsive-hyperactive; and physically aggressive, relationally aggressive and impulsive-hyperactive) ANOVA with impulsivity-hyperactivity at time 2 serving as the dependent variable. A main effect for co-morbid status was found, F(4, 89) = 6.54, P < 0.001, η _p ²  = 0.18. Bonferonni post-hoc tests indicated, not surprisingly, that the physically and relationally aggressive group that was also high on impulsivity-hyperactivity was significantly (P < 0.005) more impulsive and hyperactive at time 2 (M = 10.00; SE = 1.32) than was the nonaggressive and nonimpulsive–nonhyperactive group (M = 5.47; SE = 0.20). No other significant effects or differences emerged.