CAMBRIDGE, Mass. — Want to know how babies tell if two people are close to each other? They look at how often they swap spit. That’s the conclusion of a new study by researchers at the Massachusetts Institute of Technology, who found that infants and young children determine how close a relationship is by the pair’s saliva.

Learning to navigate social relationships is a skill that is critical for surviving in society. For babies and young children, this means that they have to learn who they can count on to take care of them. Sharing food and kissing are among the signals babies use to interpret their social world.

Neurologists from MIT identified that children use a specific signal to discover whether two people have a strong relationship, and a mutual obligation to help each other — whether they’re kissing, sharing food, or have other interactions that involve sharing saliva.

In the new study, the researchers showed that babies expect people who share saliva to help each other in distressing situations, much more so than when people share toys or interact in other ways that do not involve saliva exchange.

These findings suggest that babies can use these cues to try and work out who around them is most likely to help them.

“Babies don’t know in advance which relationships are the close and morally obligating ones, so they have to have some way of learning this by looking at what happens around them,” says senior author Rebecca Saxe, the John W. Jarve Professor of Brain and Cognitive Sciences, in a university release.

‘Thick’ relationships are what babies look for

In human societies, people are typically distinguished by having “thick” and “thin” relationships. Thick relationships, usually between family members, feature strong levels of attachment, obligation, and mutual responsiveness.

Unsurprisingly, anthropologists have observed that people in thick relationships are more willing to share bodily fluids such as saliva.

“That inspired both the question of whether infants distinguish between those types of relationships, and whether saliva sharing might be a really good cue they could use to recognize them,” lead author Dr. Ashley Tomas says.

To find the answers to those questions, the researchers observed a group of toddlers between 16.5 and 18.5 months-old and babies between eight and 10 months as they all interacted with human actors and puppets. In the first set of experiments, a puppet shared an orange with one actor, then tossed a ball back and forth with a different actor.

After the children watched these interactions, the researchers observed how the children reacted when the puppet showed distress while sitting between the two actors. Based on an earlier study of monkeys, the researchers expected that babies would first look at the person whom they expected to help.

That study showed that when baby monkeys cry, other members of the troop look to the baby’s parents as if expecting them to step in. In the new study, the MIT team found that the children were more likely to look toward the actor who shared food with the puppet, not the one who shared a toy when the puppet was in distress.

Swapping saliva is the key

In the second set of experiments, designed to focus more specifically on saliva, the actor either placed her finger in her mouth and then into the mouth of the puppet, or placed her finger on her forehead and then onto the forehead of the puppet.

Later, when the actor expressed distress while standing between the two puppets, children watching the video were more likely to look toward the puppet with whom she had shared saliva. These findings suggest that saliva sharing is likely an important cue that helps infants to learn about their own social relationships and those of people around them.

“The general skill of learning about social relationships is very useful,” Thomas says. “One reason why this distinction between thick and thin might be important for infants in particular, especially human infants, who depend on adults for longer than many other species, is that it might be a good way to figure out who else can provide the support that they depend on to survive.”

The pandemic hasn’t changed this behavior

The researchers did their first set of studies with babies who came to the lab with their families, shortly before COVID-19 lockdowns began. The team conducted later experiments over Zoom, and the results were similar before and after the pandemic — confirming that COVID-related hygiene concerns did not affect the results.

“We actually know the results would have been similar if it hadn’t been for the pandemic,” Saxe says. “You might wonder, did kids start to think very differently about sharing saliva when suddenly everybody was talking about hygiene all the time? So, for that question, it’s very useful that we had an initial data set collected before the pandemic.”

In the future, the researchers hope to perform similar studies with infants in cultures that have different types of family structures. In adult subjects, they plan to use functional magnetic resonance imaging (fMRI) scans to study what parts of the brain are involved in making saliva-based assessments about social relationships.

The study is published in the journal Science.

South West News Service writer Georgia Lambert contributed to this report.

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