Interactions that include exchanging saliva have been identified by MIT neuroscientists as a distinct signal that young children and even babies use to decide whether two individuals have a close bond and a mutual obligation to support each other.
According to a new study published in the journal Science, kissing on the mouth, sharing a spoon, and taking licks off of someone's ice cream cone are all activities that occur only when people have an especially intimate relationship, and this fact appears to be obvious to infants as young as 8 to 10 months old.
"From a really young age, without much experience at all with these things, infants are able to understand not only who is connected but how they are connected," MIT postdoc Ashley Thomas, the study's lead author, said. "They are able to distinguish between different kinds of cooperative relationships."
The findings suggest humans use saliva sharing as a cue to help define "thick" relationships. These are relationships in which people have deep bonds to one another and feel a sense of obligation to the other - and in which someone is expected to respond when the other is in distress. They are distinct from other close connections, such as friendships; they are frequently, but not always, with family.
Furthermore, the study discovered that saliva sharing was more than just how participants defined who was close to them: when they saw saliva sharing between another adult and another child (in the case of the study, a puppet), they expected that adult to provide comfort when the puppet indicated it was in distress.
These findings have piqued the interest of other researchers.
In a commentary published alongside this new study, Christine Fawcett of Uppsala University in Sweden notes that the findings not only bring to light what young children understand about the social structures around them, but also raise further questions about how children acquire these expectations and how universal they may be.
She points out that exchanging saliva with a stranger might make individuals feel disgusted, possibly as a method to protect themselves from contamination or disease, but that people will willingly do so with those close to them, even their pets.
According to Fawcett, there may be an evolutionary pressure to suppress disgust with body substances in order to aid in the care of babies, and infants' experience with this type of caretaking may then lead to a learned expectation that such behavior is connected with closeness.
Various volunteers took part in the series of experiments, but as the study progressed, the researchers recruited a more geographically, ethnically, and economically diverse population. However, all of the participants were from the U.S.
While saliva sharing may be an universal cue, Thomas pointed out that saliva norms and who is considered family vary around the world - and so may what seeing a saliva sharing connection imply.
