Past research suggests that girls, more than boys, tend to act and think in ways consistent with entity theories of personal traits: beliefs that such traits are unchangeable. This study explored how this gender difference might develop and relate to mental health problems in early adolescents across an academic year (N = 59, ages 11-14). Overall, girls endorsed stronger entity theories of thoughts, feelings, and behavior than boys. Further, girls’ entity theories grew stronger across the school year, while boys’ did not. Additional analyses suggested that entity theories were more strongly associated with mental health problems in girls than in boys. Finally, girls with greater baseline mental health problems were more likely to develop entity theories of feelings six months later.