Imagine you’re an infant. Just a few months in the world, trying to gather your faculties, testing gravity — you’re pretty helpless and fully reliant on the people around you. As a result, one of your primary tasks as a new person — even before hand-eye coordination and responding to your own name — is to figure out how the people you’ve just met are connected to each other.
“Children are born into a very social world,” said Brandon Woo, a researcher in UC Santa Barbara’s Department of Psychological & Brain Sciences. “Because it’s so important for children to work with others and collaborate with others, they need to map out these social networks.” One key piece of evidence, according to Woo and collaborators from Harvard University and MIT: saliva.
The researchers’ findings are published in the journal Evolution and Human Behavior.
Saliva is thicker than water
A developmental psychologist by training, Woo is broadly interested in life from the baby’s point of view, namely how they make sense of others’ actions, minds and relationships, all in service of understanding human sociality.
“We were inspired by studies that found that across cultures, people in close relationships are often more comfortable sharing saliva with each other,” he said. “So imagine seeing two people licking the same ice cream cone — could you guess how they might know each other?”
In previous work, Woo and collaborators found that older babies and toddlers could reliably infer how close — or “thick” — a relationship is based on intimate saliva-sharing, such as kissing, or sharing foods and utensils. “Our past work found that by the time infants are around eight months of age, when they view people sharing saliva, they are making inferences that those individuals will respond to each other’s distress,” he said.
For this work, the question wasn’t about just whether babies could infer relationship thickness; the researchers were curious about how soon they could do it. Relative to the older children, infants at four to six months of age would have had less opportunity to observe such behaviors, are less likely to look at faces, and they ingest foods that are typically not shared with others.