<p>Serotonin fosters cognitive flexibility, but how, exactly, remains unclear. We developed a computational theory that proposes that serotonin reduces belief stickiness: the tendency to get ‘stuck’ in a belief about the state of the world despite incoming contradicting evidence. We tested this theory in a randomized, double-blind, placebo-controlled study using a single dose of the selective serotonin reuptake inhibitor escitalopram. In the escitalopram group, higher escitalopram plasma levels reduced belief stickiness more, resulting in better inference about the state of the world. Moreover, participants with sufficiently high escitalopram plasma levels had less belief stickiness, and therefore better state inference, than participants on placebo. We also propose that obsessions may result from excessive belief stickiness. Indeed, participants with more obsessions had greater belief stickiness, and therefore worse state inference. The opposite relations of escitalopram and obsessions with belief stickiness may explain the therapeutic effect of selective serotonin reuptake inhibitors in obsessive–compulsive disorder.</p>