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Slobbery Secrets: Sharing Saliva Is Surprisingly Significant

Babies use the exchange of saliva as cue for judging the nature of relationships

Key points

  • Infants are not blank slates. They are biologically prepared to figure out and adapt to the natural and social worlds.
  • Infants and young children use saliva exchange to decipher the nature of social relationships in their environment.
  • Infants expect you to help someone with whom you've exchanged saliva.

Historically, psychologists used to believe that infants were not much more than brain stem preparations, “blank slates” upon which experience was to etch the contours of character. Infants, it was believed, take many years of training to develop the advanced cognitive, moral, relational skills required to function in the world.

Developmental research since the 1980s has modified these assumptions. The shift was due in part to new research methods. First, psychologists learned to take advantage of the habituation phenomenon—the fact that our nervous system activity is reduced upon repeated exposure to the same stimulus. Researchers could habituate an infant to a certain stimulus—say, a tone—and then measure what happens when that stimulus changed (for example, when the tone changes to a higher pitch). If the infant was dishabituated to the change, it meant they were able to perceive it.

Image by 947051 for Pixabay
Source: Image by 947051 for Pixabay

Second, discovering that infants will look longer at events that defy their expectations, researchers have developed sophisticated ways to track an infant's gaze. Thus, presenting infants with various scenarios and observing the direction and duration of their gaze provided a window into the processes of their minds.

In addition, psychologists have in recent years become more sophisticated about integrating insights from evolutionary science into their understanding of developmental processes. After all, infants and children face difficult survival challenges. Cognitive or behavioral adaptations that increase the chance of surviving into adulthood should be favored by natural selection and end up coded into our genetic hardware, rendering our biological slate anything but blank.

Indeed, we know now that children are born biologically prepared to survive and thrive, armed with innate equipment—human nature—the components of which have been selected over millennia for their adaptive value. Babies, for example, possess an innate numbers sense—an ability to discern differing quantities of objects—that predict later math ability. Newborn babies even show a preference for smaller numbers on the left and larger ones on the right, suggesting that this left-to-right mental number line might be innate for humans. Infants also show a knack for statistical learning, implicit learning of statistical regularities within sensory input, and demonstrate nascent logical inferences to guide their actions. What infants end up learning, in other words, they don’t have to learn from scratch.

In addition to being a (sometimes) thinking species, human beings are also social animals. Our individual survival is heavily dependent on our ability to figure out and navigate the social world. Here, too, infants come equipped with evolved tools for discerning the dynamic of their social context.

Research on attachment theory, for example, has shown how infants are born with a repertoire of proximity-seeking behaviors, enacted when they are distressed and aimed at eliciting caregiving behavior from others. Over the first months and years of life, the call-and-response dance of caregiving results in the formation of an attachment system, and with it, the child’s initial representations of self, world, and others, which form the basis for their socio-emotional future development.

Research shows that infants are attuned to an array of social cues. Infants 7- to 12-months old expect members of social groups (but not socially unrelated individuals or objects) to act alike. This expectation arises prior to language or to extensive social experience, hinting at its biological underpinnings. Infants show a preference for fairness with regard to how rewards are distributed between people; they demonstrate compassion and empathy, upon which they later construct their sense of right and wrong, their moral code. “Babies are moral animals,” according to Yale researcher Paul Bloom. By age 2, infants will help an unfamiliar adult obtain an out-of-reach object even if their parents refrain from encouraging them, or are not in the room.

A new study (2022) by Ashley Thomas of Harvard University and colleagues provides an intriguing addition to the growing literature on infants' social survival toolkit by showing that babies and young children may use, of all things, the exchange of saliva as a cue for judging the nature of relationships between people in their environment.

The authors began with the observation that “Across human societies, people form ‘thick’ relationships, characterized by strong attachments, obligations, and mutual responsiveness. People in thick relationships engage in distinctive interactions, like sharing food utensils or kissing, that involve sharing saliva.” In a series of clever experiments, the authors explored whether infants and older children may use the ‘saliva test’ to predict relationship behavior.

In the first experiment, they presented young children (5 to 7 years old) with a cartoon character shown drinking juice with a straw, and two other characters presented as the girl’s sister and her friend. They asked participants to predict whom the girl will share the juice with. Participants watching such interaction scenarios “predicted that sharing utensils, or licking the same food item, would occur within nuclear families.”

Next, the researchers tested whether toddlers and infants, upon seeing two individuals share saliva, would predict that those individuals will be more emotionally responsive to each other in future interactions. They based their design on earlier findings showing that monkeys responded to the distress of an infant by looking toward the infant’s mother. In this set of studies, the researchers showed the participants brief episodes of saliva sharing and tested whether the participants would use these cues to infer “a thick relationship” between them.

For example, in two iterations of this experiment, the researchers had their toddlers (16.5 to 18.5 months old) and infants (8.5 to 10 months old) view a puppet eating from the same orange slice with one actress (implying saliva sharing), and playing ball with another. The puppet then proceeded to sit between the two actresses, expressing distress. “Both toddlers and infants looked first, and longer, toward the actress who had shared food and saliva with the puppet.” This pattern, moreover, appeared only when the puppet showed distress and was the actress’ own “thick relation.” These results were then replicated with an economically and racially diverse sample.

Now, interaction scenarios such as those presented to the infants in this study contain many elements other than saliva exchange that may account for the infants’ response pattern. Thus, the researchers next sought to establish that it was indeed the saliva sharing aspect of the situation that compelled the infants’ responses. To that end, they showed the infants an actress interacting with a puppet. The actress put her finger in her own mouth, rotated it, and then put her finger in a puppet’s mouth, rotating it, before finally returning her finger to her own mouth. Interacting with a second puppet, the actress then performed the same routine, this time touching her own and the puppet’s forehead (rather than the mouth).

Then the researchers had the actress express distress, and measured which puppet infants and toddlers looked toward first and most. “Toddlers … looked first, and longer, towards the puppet from the mouth-to-mouth interaction, when the actress expressed distress … While Infants’ first looks were distributed equally between both puppets, they did look longer toward the puppet from the mouth-to-mouth interaction while the actress expressed distress.”

In sum, the cumulative results suggest that infants and toddlers view saliva exchange as evidence of a thick relationship and therefore “expect saliva sharing to selectively predict responses to distress.” A survey of the participants’ parents found that they, too, shared the notion that saliva sharing is appropriate in more intimate relationships.

The authors summarize: “Saliva-sharing interactions provide externally observable cues of thick relationships, and young humans can use these cues to make predictions about subsequent social interactions." And, "An early intuitive distinction between thick and thin relationships allows infants to rapidly learn the distinctive behaviors that occur in these relationships in their social environment. These rapidly bootstrapped representations would be useful for parsing the small set of thick, intimate relationships from the larger set of thin, cooperative relationships in typical human social networks.”

Saliva exchange appears to be an ancient and important, one may say sacred, ritual of human intimacy. Consider this next time your newborn grandson drools all over you (it worked for me…).

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