Why are minor keys in music sad?

"New evidence for a musical phenomenon we've taken for granted for centuries: that the minor key is sadder than the major. Dido's Lament is audibly bleaker than Kylie's I Should Be So Lucky' – although neither are as sad as the doleful monotone of the vuvuzelas that blared out from Bloemfontein on Sunday. A scientist in Massachusetts thinks she's discovered a link between the interval of a minor third (C major to E flat, say) and expressions of sadness in human speech. Meagan Curtis found in her study that the speech-melodies of actors' voices (the movement of pitch in their intonation) happened to encompass a minor third when they were asked to communicate sadness. And when listeners were played the same speech-melodies, shorn of the words, they accurately interpreted the actors' emotion."

For the full story please see The Guardian.