Infants spot sharing saliva in relationships

Infants can recognise relationships by looking for if the people involved share saliva with each other...
24 January 2022

Interview with 

Ashley Thomas, MIT


Mother & infant child in field


Children as young as 8 to 10 months old can distinguish when two people have a special - or “thick” as it’s known - relationship between them; such as being a married couple, or a parent and child. This is in comparison with close, but less special relationships, for instance between two friends. You could say this is just learned behaviour, but actually it looks like they’re using a very specific clue to work out which relationships are the important ones: if they see two other individuals sharing saliva - including by sharing food - that’s the giveaway. And when that happens, they judge those relationships to be special. Chris Smith interviews Ashley Thomas from MIT, who used puppets to test this out on human infants…

Ashley - Some friendly relationships - you can think of everyone you'd be willing to share an ice cream cone with - those are people who you feel close to, but you have other friendly relationships that you don't necessarily feel so close to, but you're still very friendly with. So that's the distinction we were after. And what we wanted to know is whether infants who have very little experience in the world distinguish between those two types of relationships.

Chris - Would this be sort of like the relationship that a parent would have with a child versus the parent's friends would have with that child?

Ashley - Yes. So that could be one place where there's a distinction, or you could think of your coworkers versus your family.

Chris - How did you set out to test this?

Ashley - We showed infants and toddlers people interacting with puppets. And in one of those interactions, we showed them saliva sharing. In another one of those interactions, we showed them something that was prosocial or cooperative, but didn't involve saliva sharing. And then we asked, given those two different interactions, who do the infants and toddlers expect to respond to the Puppet's distress or to comfort the puppet when it's upset.

Chris - When you say saliva sharing, do you mean literally?

Ashley - Yes, I do mean literally. But in this case we used food sharing. So, again, you can think of sharing an ice cream cone with someone - you sort of accidentally share saliva - but you might not be willing to do that with everybody that you know, just people who you feel close to.

Chris - What was your hypothesis then? That if saliva sharing matters they'll pay more attention to the individual with whom there had been saliva sharing rather than the one there hadn't?

Ashley - Yes, but only when the person that they had shared saliva with was upset.

Chris - Why was being upset important?

Ashley - Just like saliva sharing happens with these close relationships, so does comforting. You can also think about everyone who you would really like to comfort you when you feel sad - those tend to be people that we feel close with. And there tends to be overlap with the people who you want to comfort you and the people who you're willing to share saliva with, or share an ice cream cone with.

Chris - And is that what happened?

Ashley - That is what happened. So after infants and toddlers saw that two individuals shared saliva, they expected those two individuals to respond to one another's distress.

Chris - How does one disentangle learned experience from what's innate here?

Ashley - We can't. We tested 8 to 10 month olds who are really young and don't have that much experience in the world. But, we don't know what's innate and what's learned, but we do know that if anything's learned, then it had to have been learned really rapidly and quickly.

Chris - And does this cross cultures?

Ashley - The idea that people who are close share saliva does happen in many, many different cultures across the globe, but we don't know if infants would make the same inferences in different cultures because we haven't tested that yet.

Chris - And do you think that grownups sharing saliva or other things is part and parcel of the same underlying mechanism of trust, comfort and building relationships?

Ashley - Yeah, definitely. One thing that's cool about humans is that we can create new types of relationships or we can change a relationship that's not so close into a closer relationship. And one way that humans might communicate this is by doing these saliva sharing actions with each other.

Chris - What do you think the implications are of this?

Ashley - I think the implications are that really early on infants and toddlers are paying attention to the people around them. And they're not just paying attention to who's, who. They're paying attention to how people are connected and who's connected, and that might allow them to figure out who else would be good at taking care of them.



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