Science - USA (2022-01-21)

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It is possible that learning from one’s
own experiences to inform expectations for
others applies for the findings of Thomas et
al. as well. In one experiment, 5- to 7-year-
old children predicted that others who
shared an eating utensil or licked the same
ice cream cone were part of a nuclear fam-
ily. Further experiments examined 8.5- to
10-month-old infants’ and 16- to 18-month-
old toddlers’ intuitions about saliva shar-
ing and social behaviors. When seeing
someone in distress, these infants and tod-
dlers tended to look toward a person who
had previously been in a saliva-sharing
interaction with the distressed individual
(e.g., eating from the same orange slice),
suggesting that they expected that person
to respond and comfort, which would fol-
low if they were in a thick relationship.
Thomas et al. also collected data from a
representative US sample of parents of in-

fants and toddlers that confirmed that in-

fants are more likely to have experienced

saliva-sharing interactions with those who

are in thicker relationships with them,

such as relatives, compared with other

individuals they meet, such as friends or

nannies. Finally, an ethnographic analysis

suggested that around the world, saliva

sharing is a cue for relationship thickness

and can be used to both initiate and main-

tain close kin relationships.

In infants’ everyday experiences, caregiv-

ers frequently share food with them, kiss

them on the face, wipe away their drool

with a bare hand, and so on, whereas these

interactions are rare with noncaregiv-

ers or nonfamily members. But what is it

about saliva-sharing behaviors that make

them a reliable cue to relationship thick-

ness? The thought of exchanging saliva

with a stranger most likely elicits feelings

of disgust; however, that is not the case if

someone is a close intimate or associate—a

romantic partner, a close friend, one's own

child, or even a pet dog. It has been pro-

posed that the emotion of disgust evolved

to protect against contamination, such as

can occur when an individual comes into

contact with the bodily fluids of another

person ( 9 ). Yet taking care of an infant, for

example, requires such contact, so humans

may have also evolved an exception to the

rule: Those in a person's closest, thickest re-

lationships do not elicit disgust, no matter

the amount of drool or dirty diapers they

produce. Indeed, research has shown that

people find the smell of their own infant’s

diapers to be preferable to that of other in-

fants ( 10 ).

Because dampened disgust for close kin

provides an evolutionary explanation for

saliva-sharing behavior occurring in thick

relationships, that young children use such

behavior to infer relationship thickness

need not have direct evolutionary origins

itself. There are numerous examples of in-

fants relying on past experiences to inform

their expectations for others’ behavior, in-

cluding goal-directed actions ( 11 , 12 ), alloca-

tion of resources ( 13 ), and comforting ( 8 ).

Thus, it is reasonable to assume that the

connection between saliva sharing and thick

relationships could have become part of hu-

man social structures through learning and

cultural transmission. That is, by carrying

out certain behaviors selectively with those

closest to us, expectations and cultural tra-

ditions are established that these behaviors

indicate closeness and can even be appro-

priated to initiate closeness, as Thomas et

al. suggest from the results of their ethnog-

raphy, citing examples of cultures where sa-

liva sharing is used in wedding ceremonies

or to welcome a new infant.

Thomas et al. bring questions and con-

cepts from sociology, anthropology, and

psychology together with methods from de-

velopmental and comparative psychology.

Their findings reach across disciplinary

boundaries and provide insight into how

young children make sense of the complex

social structures around them. Yet their

work opens up further questions as well.

For example, although their ethnography

suggests that saliva sharing within thick

relationships is culturally widespread, it

is not known if the findings would gener-

alize beyond the US population that was

studied. It could be that variation in par-

enting practices across, or within, cultures

leads to variation in children’s expectations

about thick relationships. Another critical

question for future research is to clarify the

mechanisms through which young children

acquire the connection between saliva shar-

ing and relationship thickness. Thomas et

al. propose that this intuition could be a

“cognitive primitive”—something that chil-

dren are highly prepared to learn based on

evolutionary pressures ( 14 ). However, as de-

scribed above, it could rather be that evolu-

tionary pressures are only needed to shape

the behaviors that occur within thick rela-

tionships, whereas children’s sensitivity to

these connections is learned through their

experiences.

These findings not only illuminate what

young children understand about the so-

cial structures around them but also spark

further questions regarding how children

come to acquire these expectations and how

universal they might be. j

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ACKNOWLEDGMENTS

I am grateful to T. Falck-Ytter, L. Forssman, and G. Gredebäck

for their helpful comments and feedback.

10.1126/science.abn5157

21 JANUARY 2022 • VOL 375 ISSUE 6578 261

Saliva sharing, such as when

kissing or sharing food, is

a cue that young children

use to infer "thick," intimate

relationships with kin.

(^1) Uppsala Child and Baby Lab, Department of Psychology,
Uppsala University, Uppsala, Sweden.^2 Department of
Psychology, Stockholm University, Stockholm, Sweden.
Email: [email protected]