Young infants see saliva sharing as a sign of a close bond, experiments suggest‎

Imagine you're an infant. Just a few months in the world, trying to gather your faculties, testing gravity—you're pretty helpless and fully reliant on the people around you. As a result, one of your primary tasks as a new person—even before hand-eye coordination and responding to your own name—is to figure out how the people you've just met are connected to each other.