Prior research has demonstrated individual differences in children's beliefs about the stability of traits, but this focus on individuals may have masked important developmental differences. In a series of four studies, younger children (5-6 years old, Ns = 53, 32, 16, and 16, respectively) were more optimistic in their beliefs about traits than were older children (7-10 years old, Ns = 60, 32, 16, and 16, respectively) and adults (Ns = 130, 100, 48, and 48, respectively). Younger children were more likely to believe that negative traits would change in an extreme positive direction over time (Study 1) and that they could control the expression of a trait (Study 3). This was true not only for psychological traits, but also for biological traits such as missing a finger and having poor eyesight. Young children also optimistically believed that extreme positive traits would be retained over development (Study 2). Study 4 extended these findings to groups, and showed that young children believed that a majority of people can have above average future outcomes. All age groups made clear distinctions between the malleability of biological and psychological traits, believing negative biological traits to be less malleable than negative psychological traits and less subject to a person's control. Hybrid traits (such as intelligence and body weight) fell midway between these two with respect to malleability. The sources of young children's optimism and implications of this optimism for age differences in the incidence of depression are discussed.