This study examined social and temperamental influences on children's judgments about their physical abilities and relations between temperamental characteristics, ability overestimation, and accidental injuries. Six- and 8-year-olds first observed a peer succeed or fail on a set of physical tasks and then made judgments about their ability to perform those same physical tasks. At both ages, children who first watched a peer fail on the tasks made more conservative judgments about their own abilities than did children who watched the peer succeed. The relations between temperamental characteristics and judgment ability differed for the two ages. An aggregated temperament measure of Surgency/Undercontrol was related to judgment accuracy for 6-year-olds and to decision times for 8-year-olds. Likewise, the relations between temperament, ability overestimation, and accidental injuries differed for the two age groups. Ability overestimation was related to accidental injuries for 6-year-old boys whereas temperamental characteristics were related to accidental injuries for 8-year-olds. These findings suggest that both the factors that put children at risk for accidental injuries and the relations between temperamental characteristics and cognitive abilities change with development.