Babies Use Saliva Sharing to Determine Who Is Close to Whom Long Before They Can Speak

Infants Use Saliva-Sharing as a Social Compass to Map Relationships

New research suggests that infants as young as four to six months old possess a sophisticated, innate ability to identify close social bonds, specifically using saliva-sharing as a reliable cue. The findings, published in the journal Evolution and Human Behavior, indicate that babies are far more socially observant than previously assumed, mapping out their complex environment long before they master basic motor skills.

The study, led by researchers at the University of California, Santa Barbara, in collaboration with Harvard University and MIT, explores how infants categorize the strength of relationships among the adults surrounding them. According to Brandon Woo, a developmental psychologist and lead author of the study, the ability to discern who is close to whom is essential for human cooperation.

“Children are born into a very social world,” Woo says. “Because it is so important for children to work with others and collaborate with others, they need to map out these social networks.”

Decoding Social Closeness

The researchers drew inspiration from anthropological observations that across diverse cultures, intimate contact involving saliva—such as kissing or sharing food—is a universal indicator of deep social connection. Previous research by the team confirmed that older toddlers, around eight months of age, could predict that individuals who shared saliva would naturally comfort one another during times of distress.

The challenge for this latest study was determining whether younger infants, aged four to six months, also possessed this ability. At this developmental stage, infants have limited experience observing such interactions and primarily consume foods that are not typically shared.

To test this, the team designed two controlled experiments. In the first, infants observed a human actor sharing an orange slice with a puppet, a clear act of saliva exchange. This was compared to a scenario where the actor simply handed a ball to the puppet. When the puppet subsequently displayed signs of distress, the researchers monitored which actor the infants looked at longer, hypothesizing that the babies would expect the individual with the “thicker” social bond to provide help.

The second experiment involved a more abstract scenario: an actor placing a finger into their own mouth before touching a puppet, compared to a control interaction involving touching the puppet’s forehead. This allowed the team to isolate the saliva exchange from the act of sharing food.

A Foundation for Social Understanding

In both scenarios, the infants consistently looked longer at the actor who had engaged in saliva-sharing when the puppet expressed distress. This behavior suggests that the infants intuitively recognized that the saliva-sharing partners had a higher obligation to respond to the puppet’s needs.

“I think it’s really cool that babies are still making inferences in both cases,” Woo notes. “It’s fascinating that they seem to have this abstract knowledge about relationships that applies across different actions.”

The study provides significant insight into the ongoing debate regarding the cognitive starting point of human development. By demonstrating that even very young infants can identify relationship dynamics through abstract cues, the research challenges the notion that newborns begin as blank slates with no understanding of their social environment.

These findings suggest that the human brain may be pre-wired to recognize the markers of social intimacy, laying the early groundwork for how we understand human relationships throughout our lives.