A new study by Harvard and MIT researchers, including psychologist Elizabeth Spelke, shows that children as young as 8 to 10 months old infer two people are likely in a close relationship if they see them having interactions that involve a transfer of saliva.

Such activities include kissing, taking bites out of each other’s food, and sharing the same fork or straw. The study indicates that babies understand all of these activities as social cues indicating whether people are on casual terms or share stronger bonds.

The research, published in Science, also suggests babies glean from such gestures whether those involved are likelier to comfort one another if something stressful comes up — say, if one of them starts crying.

Lead researcher Ashley Thomas says the experiments involved fuzzy puppets, orange slices, and saliva-dipped fingers and were performed under the watchful eyes of infants and toddlers, ranging from 8 to 18 months of age.

The children were shown two sets of videos, some of which involved examples of saliva-sharing and some of which didn’t. In one set, a woman took a bite of an orange slice then placed it in the mouth of a fuzzy blue puppet before taking it back for another nibble. The infants and toddlers then were shown a different woman passing a ball back and forth with the puppet. The last video showed the puppet seated between both women before it starts crying and droops its head.

“The question is: Who do the infants and toddlers expect to respond to the distress of the puppet?” said Thomas, who is now a researcher at MIT but started this work as a researcher in Spelke’s Harvard lab.

The babies consistently looked first and much longer at the woman who’d shared the orange slice than the woman who just passed the ball.

As a control the two same women were also shown with a new puppet to a different group of infants and toddlers. Neither shared an orange slice with the puppet, and when the puppet started crying, the babies spent an equivalent amount of time looking at each of the two women. This indicates the determining factor for the children who looked to the woman who shared the orange slice was the relationship the babies assumed between the puppet and the woman.

Researchers also showed the babies a new set of videos with a purple and a green puppet. In these videos, a woman touched her forehead, touched the forehead of the purple puppet, then her own forehead again. The same woman then put her finger in her mouth, put it in the mouth of a green puppet, and then back in her own mouth.

The woman then sat between the puppets and acted as if in distress. The babies largely turned their focus to the puppet who’d shared saliva with the woman, signaling their expectation that it would most likely be the one to offer help.