from Caspar Eddyman at Aeon:
Spend any time with a baby and, assuming that they're fed, clean and warm, they will put most of their effort into stealing your heart. Babies are gleeful, cheerful, charismatic and gregarious-- inherent comedic qualities that ensure babies give cats a good run for their money in funny YouTube videos. Yet, while research in cognitive science has long recognised the importance of cuteness in early bonding, very few researchers have dug deeper. Until now. With a renaissance in what I like to call 'positive psychology for babies', we are starting to appreciate that smiling and laughter serve an important purpose from birth.
The survey also showed that peekaboo is the most popular way to make babies laugh the world over. It works throughout the first years of life, although the sophistication of the game can change. In the first six months, babies are genuinely surprised by your return; then they learn to anticipate it and are pleased that their predictions come true. By the time they’re toddlers, they're often playing to humour you. Throughout this period the basic elements of the game remain the same. It is all about eye contact-- which is pure social interaction, stripped down to its barest elements. Are you attending to me? Really? You can't play peekaboo while fiddling with your iPhone. There, that's better: now look me in the eye.
Peekaboo gives babies what they want more than anything: adult attention. It allows them to learn about the most confusing and compelling mystery in their world: other people. Peekaboo's popularity emphasises what appears to be the two key features of baby laughter: its purpose is to facilitate learning, and it is intensely social.
The entire article is worth a read here.