Kisses, licks, and drool: Study shows how ‘saliva sharing’ shapes babies’ understanding of the closest relationships

Raising a young child can be a bit … messy. There’s the drool to be wiped, the slobbery feeding and sharing of utensils — and plenty of kisses.

But it turns out that all that exposure to family members’ spit — what, in academic parlance, is known as “saliva sharing” — plays a crucial role in how we make sense of the world around us, a new study shows. It helps shape our discernment of social relationships, starting from our first months of life.

The study — with infants, toddlers, and young children as participants — found that we use saliva sharing as a cue to help distinguish “thick” relationships. These are connections in which people have strong attachments and feel a sense of obligation for the other — and in which someone is expected to respond when the other is in trouble. They are distinct from other close relationships, like certain friendships; often, though not always, they are with family.

Moreover, the study, published Thursday in the journal Science, determined that saliva sharing wasn’t just how the participants defined who was close to themselves: When they saw saliva sharing among another adult and another child (well, really, in the case of the study, a puppet), they expected that adult to provide comfort when the puppet signaled it was in distress.

“We’re asking how they think about relationships,” and not just their own, said developmental psychologist Ashley Thomas of Massachusetts Institute of Technology, the lead author of the paper.

Other factors beyond saliva sharing certainly inform our categorization of relationships and degrees of intimacy, Thomas said. But there’s something specific about spit. The study didn’t just include normal instances of saliva sharing, like licking the same ice cream cone or using the same straw. In one experiment, participants watched as an adult wiped the inside of her own mouth with her finger, then the inside of the puppet’s mouth, and then her own mouth again. That’s not an everyday behavior babies see — and yet they still understood it as a sign of a thick bond.

“One question developmental psychologists have been trying to answer is how infants and young children begin to parse the world into a structure that they can effectively use in order to make important decisions like whom to learn from,” Zoe Liberman, an assistant professor of psychological and brain sciences at the University of California, Santa Barbara, wrote in an email.

Liberman, who was not involved in the new study, wrote that studies have shown children pick up on close versus distant relationships, but haven’t been as clear about whether they differentiate among varying degrees of close relationship. “This work is exciting because it clearly shows at least one domain in which really young children, and even infants, are making different inferences about ‘thick’ relationships, like family members, compared to other close relationships.”

The study doesn’t address the question of whether recognizing saliva sharing as a proxy for thick relationships is something we innately know or we figure out. But some study participants were quite young, so “at the very least, they’re able to rapidly learn this connection,” Thomas said.

We’re typically turned off by others’ saliva, perhaps because we’ve evolved to be concerned about pathogens (as if we needed a reminder after two years of physical distancing). But perhaps we put that fear aside when we are dealing with those closest to us.

“Humans may have adaptations that lead them to only engage in saliva sharing with people in these ‘thick’ relationships,” Liberman wrote. That could be why it’s such an important signal to babies — they associate the people who expose themselves to their saliva (those who wipe their drool or feed them) as their caregivers.

In an editorial also published Thursday in Science, Christine Fawcett of the Uppsala Child and Baby Lab in Sweden raised a similar point.

“It has been proposed that the emotion of disgust evolved to protect us from contamination, such as can occur when coming into contact with the bodily fluids of another person,” she wrote. “Yet taking care of an infant, for example, requires such contact, so we may have also evolved an exception to the rule: Those in our closest, thickest relationships do not elicit disgust in us, no matter the amount of drool or dirty diapers they produce.”

As an example, Fawcett pointed to one study that showed parents find the smell of their own kids’ dirty diapers less disgusting than those soiled by other children.

For the new study, Thomas and developmental psychologist colleagues at Harvard and MIT had a hunch that babies might be cued by saliva sharing to determine thick relationships, a notion backed by other fields like anthropology that have found that the sharing of fluids like saliva or breast milk is a sign of intimate bonds in some cultures. The researchers then designed a series of experiments to test that hypothesis.

In one experiment, for example, children from 5 to 7 years old looking at cartoons were more likely to predict that saliva sharing actions (like drinking from the same straw) would occur with nuclear family members than with friends, whereas actions like sharing toys would happen with both family and friends.

In another experiment, infants and toddlers ranging from 8 to 18 months watched as a puppet “ate” from the same orange slice as one actress and played ball with a different actress. When the puppet then started signaling distress, the infants and toddlers looked to the orange-slice-sharing actress first and longest, “as though expecting the actress to react to the puppet’s distress,” the researchers wrote. (The experiment was modeled on prior studies that have shown that when a vervet monkey is in distress, other monkeys look to its mother to respond.)

When that puppet was swapped out for a different puppet that then expressed distress, the infants and toddlers didn’t look to the orange-slice-sharing actress first or longer compared to the ball-playing actress. These findings suggest that the children’s expectations about who would respond were tied to the saliva-sharing relationship, not whether they viewed the person as simply nice, Thomas said.

“Saliva-sharing interactions provide externally observable cues of thick relationships, and young humans can use these cues to make predictions about subsequent social interactions,” the researchers wrote.

The whole series of experiments included different participants, but as the study went on, the researchers recruited a more geographically, racially, and economically representative cohort. All the participants, however, came from the United States. While saliva sharing could be a universal cue, Thomas noted that norms around saliva and who is considered family are different around the world — and so might be what seeing a saliva-sharing relationship means.

“It could be that variation in parenting practices across, or within, cultures leads to variation in children’s expectations about thick relationships,” Fawcett wrote in the editorial.