HomeScience & TechBabies learn about the world by looking at who shares saliva

Babies learn about the world by looking at who shares saliva

THE COMPLEXITIES of human relations are difficult enough for adults to navigate—and they have at least some idea of the rules. Children have yet to learn those rules. Infants are, nonetheless, able quickly to identify close relationships between other people, and thus to build up a map of the social world around them. How they do this has perplexed sociologists, anthropologists and developmental psychologists for decades. In a paper just published in Science, Ashley Thomas of the Massachusetts Institute of Technology proposes a partial answer: slobber.

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To avoid the sexual connotations of the word “intimacy”, Dr Thomas and her team refer to the “thickness” of interactions between infants and adults—borrowing the term from Avishai Margalit, a philosopher. Thick relationships involve strong attachments, obligations and mutual responsiveness. One set of cues for thick relationships relates to things that involve sharing saliva: kissing, for example, or the common use of an eating or drinking utensil.

To test whether children interpret saliva-sharing as indicating a thick relationship, the researchers recruited two groups of several dozen youngsters. One was a set of babies aged between eight and ten months. The other was a group of toddlers aged between 16 and 18 months. To avoid the hazards of covid-19, all tests were conducted over a video link.

Each child was shown a clip of an adult interacting with a puppet, followed by a clip of that puppet in distress while the same adult, and also a stranger, looked on. When the interaction in the first clip appeared to involve the sharing of saliva—with puppet and adult portrayed as taking consecutive bites from an orange—both sets of children looked mainly at that same adult in the second clip, and not the stranger, a reaction interpreted as a belief that the adult in question would offer comfort to the puppet. When the interaction in the first clip was friendly but less thick, such as passing a ball back and forth, the children had equal expectations of both adults when shown the second clip. Saliva sharing seems, then, indicative of closeness.

That conclusion was reinforced by subsequently replacing the puppet with a different one and repeating the second test. In this case the children showed no consistent expectation about which adult would intervene to relieve the puppet’s distress. It thus seems to have been the act of sharing an orange with a specific puppet that triggered an expectation of future behaviour, rather than any inherent characteristics of the adults involved.

Conducting her experiment by video enabled Dr Thomas to cast her search for trial participants beyond Massachusetts. She nevertheless decided, in this first instance, to confine things to the United States. Future runs, she hopes, will reach beyond that country’s borders.

The ethnographic literature suggests saliva-sharing is a widespread phenomenon. It also makes sense as a signal of intimacy, for its disease-spreading potential is obvious and engaging in it therefore indicates a high degree of trust between participants. But seeing how practice varies from place to place (if, indeed, it does), might illuminate some intriguing details.

This article appeared in the Science & technology section of the print edition under the headline “Drools of attraction”

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