Kids Can Infer Close Relationships Through Saliva Sharing

As adults, we are innately aware that some activities are only okay in certain relationships. While you might get away with being drunk, swearing at high volume or farting loudly in front of close members of your family, such behavior is (usually) frowned upon during a job interview.

But how do young children grow to understand these relatively complex social arrangements and use them to navigate the world? Researchers at the McGovern Institute for Brain Research at MIT have taken an unusual approach to answering this conundrum of developmental psychology: by taking a deep dive into how children view the act of saliva sharing.  

A relation-spit made to last

The first challenge for the research team, led by Prof. Rebecca Saxe, John W Jarve (1978) professor of brain and cognitive sciences and associate dean of science at the McGovern Institute, was to define what constituted a close relationship, one that wasn’t strictly bound by genetics or romance.

“Of all the relationships you have which are positive, cooperative, helpful, mutually supportive, there is a subset that have the strongest kinds of ties and obligations,” says Saxe. The team settled on a term, used by Israeli philosopher Avishai Margalit in his book The Ethics of Memory, to describe these special bonds – thick relationships.

With this definition in hand, the team then sought a behavior unique to thick relationships. The choice of saliva-sharing, says Saxe, was based on extensive prior research. Anthropologists note that across many cultures, extremely close bonds are often described in terms of bodily substances. “We say blood brothers, right?” says Saxe. “In other cultures, it is often common to say, “they share their mother's milk,” which you could take literally, they both nursed from the same woman. But like we take “blood brothers” also to mean just very strong relationships. It's common across societies to use bodily fluids to try to describe the idea of a shared essence so deep, that it constitutes this very tight relationship.”

Over a series of experiments, Saxe’s team investigated whether kids could appreciate the subtleties of that “shared essence”. The unusual focus of the research was mirrored by an unorthodox study design that provided unexpected benefits.

Baby zoomers

In pre-COVID-19 times, Saxe explains, child psychology studies in her lab would involve asking children to come in to observe particular experimental scenarios. This approach can expose flaws in participant selection, she explained: “When our studies required babies to be able to come in person to our lab site, then of course, we were restricted to the babies in our local geographic region.”

With lab visits off the table, Saxe and team instead hosted their experiments over Zoom, an approach that paid off. “As it turns out, you can do amazing data collection over Zoom with babies, it works very well,” says Saxe. Additionally, the online element enabled the team to reach a much broader participant base, ensuring that the babies and young children in the study came from a sample representative of all households. Over 100 children were recruited in the study, where they undertook a series of experiments that revealed how they viewed behaviors appropriate in “thick” relationships.

Cartoon, puppets and saliva

An initial study highlighted that kids shown cartoons of a girl asked to choose between sharing her juice carton with a friend or sister were significantly more likely to guess that she’d share it with her family member. No such family-first differences were seen for sharing food that could be divided or toys. Saxe’s team say this implies that young children implicitly recognize that saliva-sharing occurs within close families.

In subsequent experiments, toddlers were shown videos of actresses interacting with a puppet. One actress shared an orange slice with the puppet, while the other played ball with it. When the children were subsequently shown the same puppet in distress between the two actresses, they were significantly more likely to look to the spit-sharing actress to intervene and help the puppet.

This finding was replicated in other experiments – in one, roles were reversed, so that one actress interacted with two puppets. With one puppet, the actress shared “saliva”, by putting her finger in her mouth and then the puppet’s. With the other, the actress touched her own forehead and then the puppet’s. When the actress then feigned distress, kids reliably looked first to the puppet who had shared saliva with the actress.

Extensive controls were used to validate the findings. Children didn’t look to “spit-buddy” puppets when the actress was swapped out or when no distress was shown. “The key finding,” says Saxe, “is that, like adults, toddlers and even infants use comfort and saliva sharing as a cue that two people are not just cooperative, but that their relationship is intimate. And they use this to predict responsiveness to distress.”

Born to drool

But what does this finding tell us about how babies learn to socialize? Are we simply born with a powerful desire to drool on everything and everyone we love? This would explain a lot about how babies interact with the world, after all. “So, we what we propose is that the idea that there are different types of relationships, that that could be some bias that infants are born with,” says Saxe.

Saxe explains that this idea has been widely debated in previous studies; some academics believe babies are hardwired to understand how different relationships work, whereas others believe nearly all of the distinction is learned. Saxe explains that their data could be used to support either conclusion. “And it's totally possible, with respect to our data, the baby's primitive (the information the baby is born with) is just, I should pay attention to relationships, relationships are really important,” says Saxe. If that were the case, it might be a simple matter for babies to notice that saliva sharing is a dimension of close relationships. But if all that information is learned, she points out, why has it become such an important part of cultures the world over?

“We have, in our paper, ethnographies from hundreds of years ago, that note saliva sharing rituals are important parts of key relationships. They're parts of marriage ceremonies, they're parts of welcoming new infants into a tribe. Why is the saliva sharing part of these thick relationships seen across so much of culture and time if it is just something arbitrary that babies learn from their experience?”

Perhaps in addition to babies being hardwired to seek out close relationships, says Saxe, they are also born to express those close bonds by sharing liquids from their mouth. While that might sound an incredibly specific adaptation, anyone who has seen how evolution has produced, for example, leaf-eating panda bears, can attest that it’s not always a straightforward process.

Reference: Thomas, A, Woo, B, Nettle, D, Spelke, E and Saxe, R. Early concepts of intimacy: Young humans use saliva sharing to infer close relationships. Science. 2022;375(6578): 311-315. doi: 10.1126/science.abh1054