Babies can tell who has close relationships based on one clue: Saliva

Neuroscientists have identified a specific signal that young children and even babies can use to determine whether two people have a strong relationship and a mutual obligation to help each other: whether those two people kiss, share food, or have other interactions that involve sharing saliva.

from Relationships News -- ScienceDaily https://www.sciencedaily.com/releases/2022/01/220120140732.htm