We propose a coevolutionary model of secrecy and stigmatization. According to this model, secrecy functions to conceal potential fitness costs detected in oneself or one’s genetic kin. In three studies, we found that the content of participants’ distressing secrets overlapped significantly with three domains of social information that were important for inclusive fitness and served as cues for discriminating between rewarding and unrewarding interaction partners: health, mating, and social-exchange behavior. These findings support the notion that secrecy functions primarily as a defense against stigmatization by suppressing information about oneself or one’s kin that evolutionarily has been devalued in mating and social exchange.