Saliva Sharing Might Help Kids Identify Their Closest Relationships

5:44 minutes

Baby drooling.
Baby drooling. Credit: Shutterstock

How do little kids understand who has a close relationship with them? One of the clues they use to figure it out is by noticing who they’re swapping saliva with. The closest bonds are with the people who are giving them kisses, sharing their forks, and wiping their drool. Those are the findings of a recent study published in the journal Science. 

Ira is joined by Ashley Thomas, the study’s lead author and a post doctoral fellow in the brain and cognitive sciences department at the Massachusetts Institute of Technology, in Cambridge, Massachusetts. 

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Segment Guests

Ashley Thomas

Ashley Thomas is a postdoctoral fellow in Brain and Cognitive Sciences at the Massachusetts Institute of Technology in Cambridge, Massachusetts.

Segment Transcript

IRA FLATOW: Did you ever wonder how little kids understand who has a close relationship with them? One of the clues they use to figure it out– who are they swapping saliva with? The closest bonds, it turns out, are with the people who are giving them kisses, sharing their forks, and wiping their drool.

These are the findings of a new study published in the journal Science. Joining me now to discuss her intriguing new research is Ashley Thomas, a postdoctoral fellow in Brain and Cognitive Sciences Department at MIT. Welcome to Science Friday.

ASHLEY THOMAS: Thank you. I’m really excited to be here.

IRA FLATOW: Nice to have you. This research really intrigues me. Of all the ways to research little kids’ social connections, why did you decide to study saliva?

ASHLEY THOMAS: You know, if you think about someone who you feel really close to and then another person who you really like a lot but you don’t feel that close to and then if you ask yourself, who would I be willing to share an ice cream cone with, almost everyone you ask this question to will immediately say the person they’re close to. It seems to be a cue that adults intuitively use to figure out how close two people are together.

IRA FLATOW: Hmm. And this is a cross-cultural thing. Everybody does it.

ASHLEY THOMAS: Yeah. So of course, there’s some variation. But anthropologists have found that across cultures, people who are more willing to share bodily fluids, including saliva, tend to be those who feel close to one another.

IRA FLATOW: Mm-hmm. All right. Let’s talk about little kids. How do you test if little kids were tuned in to saliva sharing as an indicator of a close connection?

ASHLEY THOMAS: Yeah. So we showed two different interactions. In one interaction, there is a woman with a puppet. And she takes a bite of an orange, feeds the puppet a bite of an orange, and then takes another bite of that orange.

In the other interaction, there’s the same puppet but a different woman. And she passes a ball back and forth with that puppet. And the question is, who will respond when that puppet gets upset?

And so we show both of the women on either side of the puppet. The puppet shakes and cries, puts its head on the table. And then we measure which woman the infants look at first and how long they look at either women, as though they expect something to happen in the video– in that area of the video where the woman is.

IRA FLATOW: And they expect the spit-bonded woman to be the one to care for her ailing companion?

ASHLEY THOMAS: Yeah. So they expect the woman who had shared food in this way that implies saliva sharing to be the one to respond.

IRA FLATOW: Huh. So why is this important for little kids to distinguish between these very close relationships?

ASHLEY THOMAS: One thing I wanted to point out is that we are not making the argument that people who don’t have these relationships aren’t good at taking care of children, because I drop my toddler off at daycare every single day. And daycare teachers are amazing.

But we don’t expect daycare teachers to have these sort of long, enduring attachments with infants. And those types of relationships are likely really important. It’s not sort of like an indicator of who’s going to necessarily in the moment be good at taking care of you. But it’s an indicator of who’s going to be around for a long time.

IRA FLATOW: So what does this study help us better understand about children’s psychological development and social relationships?

ASHLEY THOMAS: These data can’t speak to how infants are using these cues in their everyday lives because we showed them people who they had never interacted with before. What this does let us know is that there’s a lot going on in the minds of infants.

And they’re really in tune to the social interactions that are around them. And they’re using that to figure out what is this social world that I’ve been sort of thrown into that’s so complicated. Who knows each other, who’s connected, and how are they connected?

IRA FLATOW: I know you made an effort in your research to have a more representative sample of the US population. And you actually turned to TikTok to source participants in the study. Tell me more about that.

ASHLEY THOMAS: One challenge in developmental psychology is that we tend to test a really specific part of the population. It tends to be people whose parents are super educated. It tends to be overly white in terms of the makeup of the US.

And so we wanted to take this issue really seriously. We wanted to increase the diversity in terms of race, ethnicity, region in the United States, and parental income and education. And so what we did is we reached out to TikTok creators. And these TikTok creators were moms themselves. And they had followers who were moms.

And they participated with their own kid in the study. And then what they would do is make these wonderful videos that showed what it’s like to participate in a developmental psychology study and put a link to how other people could participate.

IRA FLATOW: When I read your research, it finally helped explain to me why my grandkids want to share their spit-covered food and toys with me.


ASHLEY THOMAS: Yeah. So our research can’t directly speak to how babies are using this in their own social interactions. But that is a very common observation that parents have. And one place we want to take this research in the future is to figure out how babies might be using these types of interactions in their own social experiences and within their own social relationships.

IRA FLATOW: Well, thank you very much, Ashley, for taking time to talk with us. I will now look at my granddaughter while we’re feeding it with new eyes.


ASHLEY THOMAS: Great. I’m glad.

IRA FLATOW: Ashley Thomas, postdoctoral fellow in Brain and Cognitive Sciences at the Massachusetts Institute of Technology in Cambridge, Massachusetts.

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