To investigate age-related trends in physically aggressive behaviors in children before age 2 years.

A normative US sample of 477 mothers of 6- to 24-month-old children reported on the frequency of 9 interpersonally directed aggressive child behaviors, and hurting animals, in the past month.

Almost all (94%) of the children were reported to have engaged in physically aggressive behavior in the past month. Based on 2-part regression models, the prevalences of kicking (OR, 1.70; P = .023), pushing (OR, 3.22; P < .001), and swiping (OR, 1.78; P = .018) increased with years of age, but the prevalence of hair pulling decreased with age (OR, 0.55; P = .020). The prevalences of hitting and throwing increased initially, then plateaued at age 18-20 months, and then decreased (quadratic aOR, 0.13 and 0.16; P < .001 and .010, respectively). The frequencies of hitting (R² = .05; P < .001) and throwing (R² = .03; P = .030) increased, and the frequencies of hair pulling (R² = .07; P < .001) and scratching (R² = .02; P = .042) decreased with age (P values adjusted for false discovery rate).

Physically aggressive behavior in the 6- to 24-month age range appears to be nearly ubiquitous. Most, but not all, forms of physical aggression increase with age. These results can guide pediatricians as they educate and counsel parents about their child's behavior in the first 2 years of life.