Babies learn about the world by looking at who shares saliva

THE COMPLEXITIES of human relations are difficult enough for adults to navigate—and they have at least some idea of the rules. Children have yet to learn those rules. Infants are, nonetheless, able quickly to identify close relationships between other people, and thus to build up a map of the social world around them. How they do this has perplexed sociologists, anthropologists and developmental psychologists for decades. In a paper just published in , Ashley Thomas of the Massachusetts Institute of Technology proposes a partial answer: slobber.To avoid the sexual connotations of the word “intimacy”, Dr Thomas and her team refer to the “thickness” of interactions between infants and adults—borrowing the term from Avishai Margalit, a philosopher. Thick relationships involve strong attachments, obligations and mutual responsiveness. One set of cues for thick relationships relates to things that involve sharing saliva: kissing, for example, or the common use of an eating or drinking utensil.