What makes a baby smile?

By eight months of age, babies are already smiling ’socially’. That’s according to Susan Jones and Hye-Won Hong at Indiana University who filmed 20 eight-month-old infants while they played for ten minutes alone in a room with their mother who was seated behind them. Half the time the mothers were instructed to watch their child play, the other half of the time they read a magazine and ignored their child as much as possible.

How much the infants looked at their mother didn’t depend on whether their mum was reading or not – they looked at her roughly once a minute in either case. But whether or not they smiled was affected by whether their mother was looking at them. The infants rarely smiled at their mother when she was reading, but smiled roughly 50 per cent of the time when they looked at her and she was watching. The babies rarely smiled when they were facing the toys.

In a second experiment with 16 different eight-month-olds, the researchers played back the footage to see if the mothers were doing something differently on those occasions when their infant looked at them and smiled. They couldn’t find any maternal behaviours or facial expressions that positively predicted when their child would smile at them. However, the babies were more likely to smile when they looked round at their mother after they had just been engaged in active play, as opposed to examining the toys or changing their position, for example.

Exciting toy play, a look in their mother’s direction and a social response from their mother were clearly important factors triggering a smile from the infants, the researchers concluded, yet “…none of these by itself was sufficient to produce a smile: rather the smile appeared to be a product assembled out of the positive state of the infant and a look to an attentive and, crucially, a responsive mother”, they said.

“…the finding that smiles to the toys are rare, along with the fact that a critical ingredient for most smile production is input from a social partner, underlines the degree to which smiling at eight months is a social behaviour”, Jones and Hong said.

Jones, S.S. & Hong, H-W. (2005). How some infant smiles get made. Infant Behaviour and Development, 28, 194-205.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.