A team from Harvard's Department of Psychology found that infants and young children predicted relationships by sharing saliva, and children judged whether they were members of the family core by sharing cutlery or licking the same ice cream cone. "Saliva sharing" is ubiquitous in intimate relationships, and research sheds light on how infants use external cues to understand the social structures around them.
An important task of children's development is to understand the behavior of others by stimulating their inner thoughts, beliefs, and desires ("intuitive psychology"), as well as the groups and relationships to which they belong ("intuitive sociology"). People have long debated the extent to which these intuitions can be learned from social experience, or whether they require some sort of evolutionary ability to interpret and classify behavior.
Thomas et al., of Harvard's Department of Psychology, studied children's intuition about so-called "thick" relationships—intimate relationships between people and relatives or romantic partners that are characterized by specific behaviors and obligations. The authors argue that saliva sharing between individuals is a clue used by young children to infer intimacy based on evolutionary processes that shape young children's understanding of their social environment.
Intimacy is described as an individual's body that can be seen as having a common "essence." Intimate people connect through hugs, snuggles, and kisses, and they nurture through breastfeeding, food sharing, and other acts of care. These behaviors later became external cues that could be used to infer potential intimacy. Research in growth psychology has shown that babies use behavioral cues to infer different types of social relationships. For example, they predict that people who talk to and gaze at each other will later cooperate, while people who act in sync or have similar preferences are friends.
Previous research, however, has focused on friendship or other alienated social relationships, rather than intimacy. With one notable exception, infants and young children aged 12 to 16 months want to be miserable, which is related to the attachment relationship between them and their caregivers. This suggests that infants and young children use their own experiences in relationships to predict other people's care behaviors in the first few years after birth.
The findings of Thomas et al. may also apply to learning from one's own experiences to predict the behavior of others. In one experiment, children aged 5 to 7 predicted that people who shared a cutlery or licked the same ice cream cone were members of the core family. Further experiments tested the intuition of saliva sharing and social behavior in infants aged 8.5 to 10 months and 16 to 18 months old. When seen someone in trouble, these infants tend to look for someone who has previously shared saliva with the trapped person (e.g., eating the same piece of orange), indicating that they expect the person to respond and comfort, which can happen if they are in a close relationship.
Thomas et al. also collected data on representative U.S. infant and toddler parents, confirming that babies are more likely to share saliva with people who are more intimate, such as relatives, than others, such as friends or nannies. Finally, an ethnographic analysis suggests that saliva sharing is a cue of intimacy that can be used to begin and maintain close kinship.
In infants' daily life experiences, caregivers often share food with them, kiss their faces, wipe their saliva with bare hands, etc., and these interactions are rare among non-caregivers or non-family members. But why is saliva sharing a reliable clue to intimacy? The idea of exchanging saliva with a stranger is likely to cause disgust; however, this is not the case if it's a close friend or partner — a romantic partner, a close friend, your own child, or even a pet dog.
It has been suggested that disgust has evolved to prevent pollution, as can happen when one person comes into contact with another person's bodily fluids. However, caring for babies requires such contact, so humans may have evolved an exception as well: people who are closest to each other don't cause resentment, no matter how much they drool or get their diapers dirty. In fact, studies have shown that people find that their own babies' diapers smell better than other babies' ones.
The reduction in disgust between close relatives provides an evolutionary explanation for the saliva-sharing behavior that occurs in intimate relationships, which young children use to infer intimacy in their own right that does not need to have a direct evolutionary origin. There are many examples of infants relying on past experience to predict other people's behavior, including targeted actions, resource allocation, and comfort. Therefore, it is reasonable to think that the connection between saliva sharing and intimacy through learning and cultural diffusion may have become part of the structure of human society.
Thomas et al. propose questions and concepts in sociology, anthropology, and psychology, as well as methods in growth psychology and comparative psychology. Their findings are interdisciplinary, providing insights into how young children can understand the complex social structures around them. However, the study also raises new questions. For example, although ethnographic analysis suggests that saliva sharing in intimate relationships is culturally pervasive, it is not known whether these findings apply to people outside the United States.
Another key issue for future research is to elucidate the mechanisms by which young children access the link between saliva sharing and intimacy. Thomas et al. suggest that this intuition may be a "cognitive primitive"—that children are fully prepared to learn under evolutionary pressures. However, as noted above, evolutionary pressures may only need to shape the behaviors that occur in intimate relationships, and children's sensitivities to these relationships are learned through their experiences.
These findings not only shed light on young children's understanding of the social structures around them, but also raise questions about how children receive these expectations and the universality of those expectations.
参考文献：Christine Fawcett. Kids attend to saliva sharing to infer social relationships[J]. Science,2022,375:260-261.