Spit take: Even young infants know we only swap saliva with the ones we love | Science

If you love someone, you’re probably willing to share spit with them. Whether it’s splitting an ice cream cone with your child or kissing your partner, we don’t mind trading a few germs with the people we hold dear. A new study reveals even 8-month-old children get the gist. When they see puppets or cartoon characters swapping saliva, they’re able to infer that those individuals are the most closely bonded.

“This is a stunning set of really beautiful studies,” says Lotte Thomsen, a psychologist at the University of Oslo who was not involved in the work. “The field has waited a long time for this evidence.”

Humans manage a dizzying number and variety of relationships. To survive and thrive, young children need to identify the “thickest” of these connections—the people who will nurture and safeguard them, at any personal cost. Yet when and how children master this knowledge has long puzzled scientists.

Ashley Thomas, a psychology postdoctoral researcher at the Massachusetts Institute of Technology, thought long-standing anthropology research held a clue: Studies of cultures around the world have shown people with the most intimate bonds freely share spit and other bodily fluids. She suspected young children might be picking up on the significance of these exchanges. But testing spit spreading proved hard during the COVID-19 pandemic. So she and colleagues recruited nearly 400 youngsters over Zoom, with some help from their guardians.

In the first experiment, the team asked 5- to 7-year-olds to view a series of cartoon slides featuring a child standing in a grassy field. In some cases, she was drinking from a straw in a juice box or eating ice cream; in others, she was holding a jump rope or other toy. In a second panel, a family member and a teacher or friend joined the scene. When asked who the cartoon child should share her spitty items with, volunteers picked the family member over the nonrelative 74% of the time; with nonspitty items, the volunteers were no more likely to pick the family than the nonfamily member, the team reports today in Science

A cartoon of three people standing in grass. One person says “This is the girl’s sister.” Another person says “This is the girl’s friend.”
In one experiment, young children predict whether a child would share a spitty straw with her sister or friend.Ashley Thomas

A second group of tests involved even younger participants—toddlers and infants ranging from 8 to 19 months. Here, the researchers showed the children a video of a shaggy blue puppet sharing an orange slice with one human and rolling a ball with another. The puppet then suddenly wailed and crumpled in distress. About 80% of the participants looked at the human who had shared spit with the puppet—presumably expecting this person to comfort the monster. Because the sessions were recorded on Zoom, researchers could see where the childrens’ gazes fell. The effect also held for other saliva-swapping interactions that don’t involve the already intimate act of sharing food, such as when the human put her finger in the puppet’s mouth.

a person feeds a puppet
Babies watched videos, which showed a monster puppet share an orange slice or ball with human actors.Ashley Thomas

“The fact that they found such beautiful results over Zoom … is remarkable and suggests that the cues that infants are picking up on are not at all subtle to them,” says Kiley Hamlin, a University of British Columbia, Vancouver, psychologist who studies infant cognition but was not involved in this research. “Infants are so keyed into features of their social world that might be important to their everyday lives, might be important ultimately to their survival.”

The study is “a big step in this new science of what preverbal infants already know about human sociality,” says Alan Fiske, a psychological anthropologist at the University of California, Los Angeles, who first hypothesized the link between body fluid exchanges and close human ties. He points out, however, that these deep bonds can be inferred from other behaviors, such as sharing a bed, cuddling, and intimate touch. In other words, he says, spit sharing isn’t the only way babies know who loves them.