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Based on one indication, babies can determine who has close relationships: Saliva


Sharing food and kissing are two of the cues that newborns use to assess their social environment.

Neuroscientists have discovered a precise signal that young toddlers and even newborns may use to decide if two people have a deep connection and a shared commitment to assist each other: whether those two individuals kiss, share food, or engage in other behaviors that include saliva exchange.

Learning to negotiate social interactions is a necessary skill for survival in human cultures. For newborns and young children, this entails figuring out who they can trust to look after them.

MIT neuroscientists have discovered a precise signal that young toddlers and even newborns use to decide if two people have a deep bond and a reciprocal commitment to assist one other: whether those two individuals kiss, share food, or engage in other behaviors that include saliva exchange.

In a recent study, researchers discovered that newborns anticipate individuals who share saliva to come to each other’s rescue when one of them is in distress, far more than when adults share toys or engage in other ways that do not require saliva interaction. According to the researchers, the results show that newborns may utilize these signals to attempt to figure out who around them is most likely to give assistance.

“Babies don’t know which relationships are close and morally obligatory, so they have to have some way of learning this by looking at what happens around them,” says Rebecca Saxe, the John W. Jarve Professor of Brain and Cognitive Sciences, a member of MIT’s McGovern Institute for Brain Research, and the study’s senior author.

Ashley Thomas, an MIT postdoctoral researcher, is the study’s primary author, and it was published today in Science. The research is also co-authored by Brandon Woo, a graduate student at Harvard, Daniel Nettle, a professor of behavioral science at Newcastle University, and Elizabeth Spelke, a professor of psychology at Harvard.

Saliva sharing

People in human civilizations usually discriminate between “thick” and “thin” connections. Thick connections, which are often seen amongst family members, are characterized by high degrees of attachment, duty, and reciprocal response. Anthropologists have also shown that individuals who are in close relationships are more eager to exchange body fluids like saliva.

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“That sparked the issue of whether newborns discern between those sorts of interactions and whether saliva sharing may be a very excellent indicator they could use to detect them,” Thomas adds.

The researchers examined toddlers (16.5 to 18.5 months) and newborns (8.5 to 10 months) while they viewed interactions between human performers and puppets. In the first series of studies, a puppet shared an orange with one actor before throwing a ball back and forth with another.

Following the observation of the children’s responses when the puppet shown anguish while sitting between the two actors, the researchers observed the children’s reactions when the puppet displayed distress while sitting between the two actors. Based on previous studies with nonhuman primates, the researchers assumed that newborns would glance first at the person they expected to assist. When infant monkeys cry, other members of the group turn to the baby’s parents, as though expecting them to intervene.

When the puppet was in distress, the youngsters were more inclined to turn toward the actor who had shared food with the puppet rather than the one who had shared a toy, according to the MIT researchers.

In a second series of studies, the actress either inserted her finger in her mouth and subsequently into the mouth of the puppet, or she placed her finger on her forehead and then onto the forehead of the puppet. Children viewing the film were more likely to turn toward the puppet with whom she had exchanged saliva when the actress indicated anguish while standing between the two puppets.

Cues from others

According to the researchers, the results indicate that saliva sharing is likely a key indication that helps newborns learn about their own social interactions as well as those of those around them.

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“Learning about social interactions is an universal skill that is really beneficial,” Thomas explains. “One reason why this distinction between thick and thin may be important for infants in particular, especially human infants, who rely on adults for a longer period of time than many other species, is that it may be a good way to figure out who else can provide the support that they rely on to survive.”

The researchers conducted their initial set of trials with newborns who came to the lab with their families just before the Covid-19 lockdowns started. Later trials were conducted through Zoom. The researchers saw identical findings before and after the epidemic, demonstrating that pandemic-related hygiene concerns had no effect on the outcome.

“We really know that if it hadn’t been for the epidemic, the outcomes would have been comparable,” Saxe explains. “You could question whether youngsters changed their minds about sharing saliva when everyone started preaching about cleanliness all the time. So, for that reason, the fact that we had an initial data set acquired before to the pandemic is quite helpful.”

Because the participants were not confined to families who could come to the lab in Cambridge during regular working hours, the researchers were able to enroll a considerably more varied group of youngsters for the second series of tests on Zoom.

The researchers intend to do comparable experiments with newborns in cultures with diverse sorts of family systems in the future. They want to employ functional magnetic resonance imaging (fMRI) on adult people to investigate which areas of the brain are involved in forming saliva-based evaluations about social ties.

The National Institutes of Health, the Patrick J. McGovern Foundation, the Guggenheim Foundation, a Social Sciences and Humanities Research Council Doctoral Fellowship, MIT’s Center for Brains, Minds, and Machines, and the Siegel Foundation all contributed to the study.

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