By nine months of age, babies can already tell the difference between jolly jingles and sad ones. You can probably imagine that demonstrating this was no mean feat for researchers, given the obvious difficulties of asking babies what they think.
Ross Flom and colleagues took advantage of the fact that babies tend to look longer at something that's novel. Of course, this depends on their ability to tell that something is new and different.
Dozens of babies aged between three and nine months were presented with a video image of a male or female actor with a neutral facial expression. Musical excerpts were played through speakers located near this face.
Each experimental trial always began with all happy or all sad music. After a while the babies stopped looking for so long in the direction of the face and music - they "habituated" to it. Soon afterwards, the researchers changed the music. If it had been happy at the start, they changed it to sad, and vice versa.
For three-month-olds, changing the mood of the music made no difference - they were still bored by it and didn't look much in the direction of the face and music. By contrast, for nine-month-olds, changing the mood of the music grabbed their attention. They realised it was different and started looking in the direction of the face and music more often. The results for five and seven-month-olds were mixed. A switch from sad to happy music grabbed their attention, but from happy to sad did not - the researchers aren't entirely sure why this is, but it may have something to do with sad music being inherently less interesting.
A couple of control conditions made the results more persuasive. Firstly, the 3-month-olds began looking more in the direction of the music if the display changed to show a spinning turtle - so their lack of a reaction to the musical change can't have been due to fatigue. Also, the attention of the older babies wasn't grabbed simply by playing a new piece of music of the same mood - the mood had to change.
Although the older babies recognised a change in the mood of the music, it's not clear how much this really meant to them. "We make no claims about whether infants perceived affect in the music or experienced either happiness or sadness while listening to it," the researchers said.
R FLOM, D GENTILE, A PICK (2008). Infants’ discrimination of happy and sad music Infant Behavior and Development, 31 (4), 716-728 DOI: 10.1016/j.infbeh.2008.04.004