science.org SCIENCEPHOTO: IGOR ALECSANDER/GETTY IMAGES260 21 JANUARY 2022 • VOL 375 ISSUE 6578By Christine Fawcett1,2O
ne of the great tasks of child devel-
opment is to make sense of other
people’s behavior by appealing to
their internal thoughts, beliefs, and
desires [“intuitive psychology” ( 1 )],
as well as the groups and relation-
ships that they are a part of [“intuitive so-
ciology” ( 2 )]. There has long been debate
over the extent to which these intuitions
can be learned from social experience or
whether they require some evolved capac-
ity to interpret and categorize behavior. On
page 311 of this issue, Thomas et al. ( 3 ) ex-amine children’s intuitions about so-called
“thick” relationships—intimate bonds that
people have with kin or romantic partners
that are characterized by certain behaviors
and obligations ( 4 ). The authors suggest
that saliva sharing between individuals is a
cue that young children use to infer thick
relationships, and that these inferences are
based on evolutionary processes that have
shaped how young children interpret the
social world.
Thick relationships have been described
as those in which individuals’ physical bod-
ies can be seen as sharing a common “es-
sence” ( 4 ). Thickly related individuals cometogether in hugging, cuddling, and kissing,
and they nurture through breastfeeding,
food sharing, and other caretaking behav-
iors. These behaviors then become outward
cues that can be used to infer the underly-
ing thick relationship. Developmental psy-
chology research has shown that infants
use behavioral cues to infer different kinds
of social relationships. For example, they
expect that people who talk to and look
at each other will later cooperate ( 5 ) and
that people who move in synchrony ( 6 ) or
have similar preferences ( 7 ) are friends.
However, these previous studies have pri-
marily examined friendship or other thin
social relationships, rather than thick ones.
In one notable exception, 12- to 16-month-
old infants’ expectations for a distressed
individual to be approached by a caregiver
were related to their attachment relation-
ship with their own caregiver ( 8 ). This sug-
gests that already in the first years of life,
infants use their own experiences in rela-
tionships to make predictions about others’
caregiving behaviors.PERSPECTIVES
INSIGHTS
DEVELOPMENTAL PSYCHOLOGYKids attend to saliva sharing to
infer social relationships
Saliva sharing suggests “thick,” intimate bonds