Past research has shown that hostile schemas and adverse experiences predict the hostile attributional bias. This research proposes that seemingly nonhostile beliefs (implicit theories about the malleability of personality) may also play a role in shaping it. Study 1 meta-analytically summarized 11 original tests of this hypothesis (N = 1,659), and showed that among diverse adolescents aged 13–16 a fixed or entity theory about personality traits predicted greater hostile attributional biases, which mediated an effect on aggressive desires. Study 2 experimentally changed adolescents' implicit theories toward a malleable or incremental view and showed a reduction in hostile intent attributions. Study 3 delivered an incremental theory intervention that reduced hostile intent attributions and aggressive desires over an 8-month period.