Longitudinal studies of small samples of preschool children have found that the most physically aggressive toddlers tend to remain the most aggressive preschoolers; however, mothers' reports of their children's physical aggression from the first wave of the National Longitudinal Survey of Children and Youth indicate that the mean frequency of physical aggression peaks at the end of the second year after birth and then steadily declines. Studies have shown that by 12 months of age, children have the physical, cognitive, and emotional means of being physically aggressive toward others. It appears that most children will at some point hit, bite, or kick another child or even an adult. Children's individual characteristics can explain part of the variance in the frequency and stability of this behavior, but the quality of children's relations with their environment and the environment's reactions to this behavior will very likely be important factors. If children are surrounded by adults and other children who are physically aggressive, they will probably learn that physical aggression is part of daily social interactions. On the other hand, if a child lives in an environment that does not tolerate physical aggression and rewards prosocial behavior, it is likely that the child will acquire the habit of using means other than physical aggression to obtain what he/she wants or for expressing frustration. 4 figures and 32 notes