Babies can tell who’s closely related from whether they share saliva

Infants and toddlers seem to expect people who exchange saliva, for example by taking bites of the same food, to be close enough to comfort each other if one gets upset

By Alice Klein

Babies and toddlers can identify people who are intimately related based on whether they exchange saliva, which may help them to understand the social world around them.

Young children and their caregivers often share saliva, for example, if they kiss on the lips or eat off the same spoon. As a result, children may learn that saliva-sharing is a sign of close relationships.

To test this idea, Ashley Thomas at Harvard University and her colleagues showed babies and toddlers videos of puppets and actors in different scenarios.

In one experiment, 20 babies aged between 8.5 and 10 months old and 26 toddlers aged 16.5 to 18.5 months watched a puppet eating from the same orange slice as a female actor – implying saliva-sharing – and playing ball with another. When the puppet later began to cry, the babies and toddlers tended to look first and for longer at the saliva-sharing actor, as if they assumed she was more likely to provide comfort.

The experiment was repeated with 118 US toddlers aged 14.5 to 19 months from diverse racial, economic and geographical backgrounds and produced the same results.

In another experiment, toddlers watched an actor put her finger in her mouth and then in the mouth of a puppet. They also watched her touch her forehead and then touch the forehead of another puppet. When the actor later looked distressed and said “oh no”, the toddlers looked first and for longer at the saliva-sharing puppet, as if expecting it to be the main comforter.

Together, these results suggest that young children pick up saliva-sharing cues and use them to identify “thick” relationships – those that involve strong attachments and moral obligations of care, such as between close family members, says Thomas.

It is important for young children to know who they have thick relationships with because they are dependent on others for survival and must figure out who is most likely to respond to their needs and distress, says Thomas.

“The pattern of who does, and who does not, share saliva may help infants to distinguish those who are kin (e.g. parents, siblings, grandparents) versus non-kin (e.g. daycare teachers, nannies) among their many caregivers,” she says.

Being able to perceive the level of closeness between unfamiliar individuals is also useful because it allows children to “make sense of the complex social structures around them”, writes Christine Fawcett at Uppsala University in Sweden in a commentary piece accompanying the study.

Saliva-sharing is one behaviour that children interpret as a sign of a close relationship but there are probably many others, like cuddling and providing emotional comfort, says Thomas.

Her team now wants to study how babies and toddlers apply their understanding of close relationships. For example, they plan to test if infants prefer to take food from their parent’s close partners over their less intimate friends, she says.

Journal reference: Science, DOI: 10.1126/science.abh1054