Early in life, babies and toddlers need to figure out who will nurture them and keep them safe. Older kids are more verbal and better able to distinguish between their relationships, but babies and preverbal toddlers can’t understand so readily which relationships are the close ones they can depend upon. Instead, they are hard-wired to understand how certain interactions imply different categories of relationships.

Babies and toddlers produce a lot of fluids along the way to mastering their bodies. They also watch everything we do—and usually want a drooly bite of everything we eat. 

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Through thick and thin

In a new study from Massachusetts Institute of Technology (MIT), researchers wanted to demonstrate that distinctive behaviors that happen between babies and toddlers and the adults in their world help them know intuitively who is or isn’t family, thus who is more likely to take care of them. 

Researchers posited that babies and toddlers might be making a distinction among those who do or do not share saliva, as a determinant for who is a “thick” relation (like family, parents, siblings and grandparents) or “thin” relation (other adults, like daycare providers and nannies). Family members usually have strong levels of attachment, obligation and mutual responsiveness—and according to anthropologists, more willingness to share bodily fluids, such as saliva.

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“One reason why this distinction between thick and thin might be important for infants in particular, especially human infants, who depend on adults for longer than many other species, is that it might be a good way to figure out who else can provide the support that they depend on to survive,” explains lead researcher, Ashley Thomas, a psychology postdoctoral researcher at MIT who studies what babies and young children understand about the complexities of their social world. “That inspired both the question of whether infants distinguish between those types of relationships, and whether saliva sharing might be a really good cue they could use to recognize them,” Thomas said in MIT News. 

For babies, sharing saliva equates to intimacy

The researchers at MIT carefully designed a series of experiments to determine if babies and toddlers expect people who share saliva to help each other when one is in distress, much more so than when people share toys or interact in other ways that do not involve sharing spit. 

In their experiments, the researchers observed almost 300 toddlers (16.5 to 18.5 months) and babies (8.5 to 10 months) as they watched brief videos depicting saliva sharing and other activities between women actors and puppets. 

In one video, a woman rolled a ball back and forth with a blue puppet. Then a different woman put a slice of orange in her mouth before giving the blue puppet a nibble of the slice and then putting it back in her mouth to finish it, demonstrating that the woman and blue puppet shared saliva. 

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Then the puppet cried—and when it did, almost 4 out of 5 babies and toddlers looked first and looked longer at the woman who had shared a bite of her orange—and spit—with the puppet, expecting this person to comfort the puppet. “Both of these interactions are perfectly friendly and pro-social,” says Thomas, “but taking bites off the same food suggests a more intimate relationship than simply playing ball.”

To make sure that it wasn’t just the sharing of food that seemed to make babies and toddlers think there was a ”thick” connection, the researchers created another video that showed a different woman sharing just her saliva. In the video, she put her finger in her mouth and then put it into a purple puppet’s mouth, and then put it back in her own mouth.

When the two women were depicted with this new purple puppet as it started to cry, infants and toddlers looked at both women equally often. This suggested to the researchers that the babies and toddlers didn’t perceive the particular food-sharing woman as especially helpful since both women had shared saliva with the puppets. Instead, the researchers surmised that it was the relationship with the puppet that really mattered.

“Saliva-sharing interactions provide externally observable cues of thick relationships, and young humans can use these cues to make predictions about subsequent social interactions,” the researchers report. The findings suggest that saliva helps babies and toddlers to quickly identify who of those around them are in the “thick” category of relationships in order to figure out who is most likely to offer help, and that they can distinguish closeness very early in life. 


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