"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 test whether infants made this distinction, the video then shows the two women with the blue fuzzy puppet in between them. The puppet starts to cry and puts its head down, as if it is suddenly unhappy.
When the puppet cried, infants and toddlers looked first and looked longer at the woman who had shared bites of her orange.
"They're looking in that direction because they expect something to happen there," says Thomas. "They expect that woman to be the one to respond to the puppet's distress."
When the two women were shown with a totally new puppet that started to cry, however, infants and young children looked at both women equally often. This suggested that they didn't see this particular food-sharing woman as especially helpful; instead, her relationship with the puppet was what really mattered.
To make sure it wasn't just sharing of food that seemed to make babies infer the existence of a close social connection, the researchers created another, similar video. This time, instead of sharing an orange slice, a woman simply put her finger in her own mouth and then put it in a purple puppet's mouth, before putting it back in her own mouth.
Then that same woman also interacted with a green puppet, touching its forehead and then touching her own forehead. After that, the video showed the woman seemingly in distress, with the purple and green puppets looking on.
Infants and toddlers gazed at the purple puppet that had the more intimate, finger-in-the-mouth interaction, as if expecting this puppet to be more affected by the woman's consternation, presumably because they seemed to have a closer relationship.
The researchers also studied older children, aged 5 to 7, and told them about another child who was sharing stuff. Some of the sharing involved contact with saliva, although the scientists never explicitly referred to spit.
"We just said, like, 'This kid is eating applesauce with a spoon and he shares his applesauce with one of these two people using his spoon. Who do you think he shared with?' And the choices were always between a family member and a friend," explains Thomas.
For items that could be easily divvied up, like separate pieces of candy or toys, kids thought a person was just as likely to share with a friend as a family member.
"But when it comes to saliva-sharing items, like sharing an ice cream cone or using the same spoon, then kids think that the kid is more likely to share with family," says Thomas.
Saliva as social glue
Other researchers find these results intriguing.
"These findings not only illuminate what young children understand about the social structures around them but also spark further questions regarding how children come to acquire these expectations and how universal they might be," writes Christine Fawcett of Uppsala University in Sweden, in a commentary that was published along with this new study.
She notes that the idea of exchanging saliva with a stranger can create feelings of disgust, perhaps as a way to protect people from contamination or disease, but that people will happily do this with those close to them, even pet dogs.
There could be an evolutionary pressure to suppress disgust with bodily substances to aid in taking care of babies, and infants' experience of this kind of caretaking could then lead to a learned expectation that such behavior is associated with closeness, Fawcett points out.
But Alan Fiske, an anthropologist at UCLA, believes that babies have an innate understanding of certain kinds of social relationships. He's written that humans are born primed to recognize four fundamental forms of relationships, and he calls this study "enormously important."
In relationships characterized by "communal sharing," he says, sharing saliva "is a way of connecting bodies, or making bodies the same in some respect. And that's the crucial thing. When people feel that some how they are essentially the same, almost in an embodied way, then they feel socially the same."
"Spit sharing is one instance of, or one type of, connecting bodies physically through bodily substances," says Fiske. But there are other ways — such as having sex, breastfeeding, or even mingling blood to become "blood brothers." The ritual of communion in Christianity, he notes, involves ingesting the body and blood of Jesus Christ as a way for that religion to express and reinforce a communal sharing relationship.
This kind of close relationship can be created between people in other ways that don't involve body fluids, however, such as grooming, snuggling and hugging, or synchronous rhythmic movement such as dancing or marching, says Fiske.
In this view, babies seem to just know all this innately. He believes that future studies will show that babies not only observe these activities to understand the social links of those around them, but also actively initiate these behaviors themselves in order to forge relationships with others.