MIT neuroscientists have now identified a specific signal that young children and even babies use to determine whether two people have a strong relationship and a mutual obligation to help each other.
“Babies don’t know in advance which relationships are close and morally obligating ones, 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 senior author of the new study.
In human societies, people typically distinguish between “thick” and “thin” relationships. Thick relationships, usually found between family members, feature strong levels of attachment, obligation, and mutual responsiveness.
Anthropologists have also observed that people in thick relationships are more willing to share bodily fluids such as saliva.
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.
To study those questions, researchers observed toddlers (16.5 to 18.5 months) and babies (8.5 to 10 months) as they watched interactions between human actors and puppets.
In the first set of experiments, a puppet shared an orange with one actor, then tossed a ball back and forth with a different actor.
After the children watched these initial interactions, the researchers observed the children’s reactions when the puppet showed distress while sitting between the two actors.
Researchers found that the children were more likely to look toward the actor who had shared food with the puppet, not the one who had shared a toy when the puppet was in distress.
In the second set of experiments, designed to focus more specifically on saliva, the actor either placed her finger in her mouth and then into the mouth of the puppet, or placed her finger on her forehead and then onto the forehead of the puppet.
Later, when the actor expressed distress while standing between the two puppets, children watching the video were more likely to look toward the puppet with whom she had shared saliva.
These findings suggest that saliva sharing is an important cue that helps infants to learn about their social relationships and those of people around them.
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.
In future work, researchers hope to perform similar studies with infants in cultures that have different types of family structures.
In adult subjects, they plan to use functional magnetic resonance imaging (fMRI) to study what parts of the brain are involved in making saliva-based assessments about social relationships.