Two people pass a ball. Two others take turns eating the same orange slice. Both interactions look friendly—but to a baby, they may signal different degrees of closeness.
A 2022 study found that infants and toddlers watched these interactions differently. When a puppet later appeared distressed, they looked toward the person who had shared its food rather than the person who had merely played with it. Researchers interpreted this as an expectation that the closer partner would provide comfort. Study pubmed.ncbi.nlm.nih.gov
The proposed clue was saliva sharing: an interaction associated with intimate relationships through kissing, shared utensils and shared bites.
A 2025 replication with 50 infants provided partial support. Babies looked longer toward the food-sharing partner, but did not reliably look there first. The researchers also raised an alternative: infants might recognize feeding as evidence of care, without attending specifically to saliva. Replication PMC
Supported: infants can use observed interactions to distinguish social relationships. The specific role of saliva remains less certain.
Plausible but debated: natural selection favored sensitivity to intimate interactions because dependent infants benefited from identifying dependable caregivers. These experiments cannot establish that evolutionary history, or separate inherited expectations from early learning.
Before babies understand words such as “family” or “close friend,” they may already be gathering clues about something essential: who is likely to show up when someone needs help.
