Most people, at least in Western culture, are likely to say that they listen to music not because it gives them some abstract mental stimulation but because it moves them. As Leo Tolstoy put it, 'music is the shorthand of emotion'. This emotional response seems to operate beyond volition: we might be fully aware, when the sad strings swell up in the soundtrack to a movie, that the movie itself is cheap and sentimental and that we are being manipulated shamelessly, but that doesn't mean we can hold back the tears.

How does music acquire this sort of power? Why can it make us laugh and weep and dance? In the answer to this question surely lies one of the key reasons for why we listen to music. But it's a tremendously difficult question to answer. It is hard enough to know what we mean by emotion in the first place, let alone what that means in relation to music. Does sad music actually make us feel sad, or do we just recognize a quality of sadness in it? Some broad emotional qualities such as happiness and sadness seem to be recognized in music across cultures, probably because they mimic the appearances and behaviour of people who feel that way. But rather little music is simply and consistently happy or sad, or is sad or happy in the same way.

The main candidate for a more general theory of musical emotion suggests that this arises when music sets up expectations for what will happen next, and then either violates or postpones fulfilment of that expectation. It seems to explain a lot about how music is both composed and performed – for a performance is generally thought to be expressive when it deviates just the right amount from precise tuning or timing. But this is not the whole story.