Early concepts of intimacy: young humans use saliva sharing to infer close relationships

Across human societies, people form ‘thick’ relationships, characterized by strong attachments, obligations and mutual responsiveness. People in thick relationships engage in distinctive interactions, like sharing food utensils or kissing, that involve sharing saliva. Here we show that children, toddlers, and infants infer that dyads who share saliva (compared to other positive social interactions) have a distinct relationship. Children expect saliva sharing to happen in nuclear families. Toddlers and infants expect that people who shared saliva will respond to one another in distress. Parents confirm that saliva sharing is a valid cue of relationship thickness in their children’s social environments. The ability to use distinctive interactions to infer categories of relationships thus emerges early in life, without explicit teaching, allowing young humans to rapidly identify close relationships, both within and beyond families.