BACKGROUND Certain disorders, such as depression and anxiety, to which serotonin dysfunction is historically associated, are also associated with lower assessments of other people's trustworthiness. Serotonergic changes are known to alter cognitive responses to threatening stimuli. This effect may manifest socially as reduced apparent trustworthiness of others. Trustworthiness judgments can emerge from either direct observation or references provided by third parties. OBJECTIVE We assessed whether explicit judgments of trustworthiness and social influences on those judgments are altered by changes within serotonergic systems. METHODS We implemented a double-blind between-subject design where 20 healthy female volunteers received a single dose of the selective serotonin reuptake inhibitor (SSRI) citalopram (2 × 20 mg), while 20 control subjects (matched on age, intelligence, and years of education) received a placebo. Subjects performed a face-rating task assessing how trustworthy they found 153 unfamiliar others (targets). After each rating, the subjects were told how other subjects, on average, rated the same target. The subjects then performed 30 min of distractor tasks before, unexpectedly, being asked to rate all 153 faces again, in a random order. RESULTS Compared to subjects receiving a placebo, subjects receiving citalopram rated targets as less trustworthy. They also conformed more to opinions of others, when others rated targets to be even less trustworthy than subjects had initially indicated. The two effects were independent of negative effects of citalopram on subjective state. CONCLUSIONS This is evidence that serotonin systems can mediate explicit assessment and social learning of the trustworthiness of others.