Impressions about harm are formed rapidly and then refined, modulated by serotonin.

Attributing motives to others is a crucial aspect of mentalizing, can be biased by prejudice, and is affected by common psychiatric disorders. It is therefore important to understand in depth the mechanisms underpinning it. Toward improving models of mentalizing motives, we hypothesized that people quickly infer whether other's motives are likely beneficial or detrimental, then refine their judgment (classify-refine). To test this, we used a modified Dictator game, a game theoretic task, where participants judged the likelihood of intent to harm vs. self-interest in economic decisions. Toward testing the role of serotonin in judgments of intent to harm, we delivered the task in a week-long, placebo vs. citalopram study. Computational model comparison provided clear evidence for the superiority of classify-refine models over traditional ones, strongly supporting the central hypothesis. Further, while citalopram helped refine attributions about motives through learning, it did not induce more positive initial inferences about others' motives. Finally, model comparison indicated a minimal role for racial bias within economic decisions for the large majority of our sample. Overall, these results support a proposal that classify-refine social cognition is adaptive, although relevant mechanisms of serotonergic antidepressant action will need to be studied over longer time spans.