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Encore: Babies and toddlers know that swapping saliva is a sure sign of love

STEVE INSKEEP, HOST:

You maybe know that saying, it ain't worth spit. Well, this story is worth spit because it's about spit. It's about people who share saliva, something you do if you eat off the same spoon or kiss. It's usually a sign that two people are emotionally close. Babies are fountains of drool, so a study examined their views of sharing saliva. Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: Did you ever see that "Seinfeld" episode where Elaine likes a guy and wants to ask him out on a date?

(SOUNDBITE OF TV SHOW, "SEINFELD")

JULIA LOUIS-DREYFUS: (As Elaine Benes) But then he wiped his hand on the top of the bottle when I offered him water.

(LAUGHTER)

JASON ALEXANDER: (As George Costanza) Well, that doesn't mean anything.

LOUIS-DREYFUS: (As Elaine Benes) Are you kidding? That's very significant. If he was interested in me, he'd want my germs. He'd just crave my germs.

GREENFIELDBOYCE: It's only funny because it's true.

ASHLEY THOMAS: And the question is sort of, well, how do you know that?

GREENFIELDBOYCE: Ashley Thomas is a researcher at MIT.

THOMAS: Is it something that you know because of years and years of experience with these types of interactions? Or is it something that even infants and toddlers, who have very little experience with these types of interactions, also know?

GREENFIELDBOYCE: To find out, she and some colleagues tested babies and toddlers. The kiddos watched videos of puppet shows. In one, a woman rolls a ball back and forth with a blue, fuzzy puppet. Then another woman feeds the puppet.

THOMAS: By taking a bite of an orange slice and letting the puppet take a bite of the orange slice and then taking another bite of that same orange slice.

GREENFIELDBOYCE: At the end of the video, the puppet is between these two women and starts to cry.

(SOUNDBITE OF BABY CRYING)

GREENFIELDBOYCE: And it turns out infants and toddlers looked first and looked longer at the woman who'd shared food and saliva. It's like they expected her to do something. That was intriguing, but maybe what mattered was sharing food, not saliva. So the researchers did a study with no food. This time, two different puppets interacted with one woman. The woman shared saliva with only one of the puppets.

THOMAS: The saliva-sharing interaction involved the woman putting her finger in her mouth, putting her finger in the puppet's mouth and then putting her finger back in her mouth.

GREENFIELDBOYCE: With the other puppet, she just touched its forehead and touched her own forehead. Later on, when the woman appeared to be unhappy...

THOMAS: We find that they expect the puppet who had had that mouth-to-mouth interaction to be the one to respond to the woman's distress.

GREENFIELDBOYCE: The researchers report these findings in the journal Science along with the results of another experiment in older children. Five-to-7-year-olds were told about a kid who was sharing stuff - food, toys. And they were asked who the kid was more likely to share with, a friend or a family member. For stuff that could easily be divvied up, there was no difference.

THOMAS: 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.

GREENFIELDBOYCE: Of course, older kids are often explicitly told not to swap saliva with others, especially during this pandemic. But how do babies get clued into all this? Alan Fiske is an anthropologist at UCLA. He believes babies are born primed to understand certain basic kinds of human relationships. In one of them, people are socially and functionally equivalent, and saliva-sharing signals that.

ALAN FISKE: It is a way of connecting bodies or making bodies the same in some respect. And that's the crucial thing.

GREENFIELDBOYCE: He says there's other ways to make bodies feel the same, like close cuddling or sex or even ritually becoming blood brothers, so there's nothing magic about spit.

Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF WILD NOTHING SONG, "PARADISE") Transcript provided by NPR, Copyright NPR.