The mere thought effect is defined in part by the tendency of self-reflective thought to heighten the generation of and reflection on attitude-consistent thoughts. By focusing on individuals’ fears of invalidity, we explored the possibility that the mere opportunity for thought sometimes motivates reflection on attitude-inconsistent thoughts. Across three experiments, dispositional and situational fear of invalidity was shown to heighten reflection on attitude-inconsistent thoughts. This heightened reflection, in turn, interacted with individuals’ thought confidence to determine whether attitude-inconsistent thoughts were assimilated or refuted and consequently whether individuals’ attitudes and behavioral intentions depolarized or polarized following a sufficient opportunity for thought, respectively. These findings emphasize the impact of motivational influences on thought reflection and generation, the importance of thought confidence in the assimilation and refutation of self-generated thought, and the dynamic means by which the mere thought bias can impact self-persuasion.