Alan Marsden in Scientific American:
New research published in the journal Royal Society Open Science attempts to tackle this issue by investigating the links between the emotions of lyrics and the musical elements they are set to. While the methods used are sophisticatedly statistical, the conclusions are extremely dry. The finding that a single chord type is most associated with positive lyrics is a huge simplification of the way that music works, highlighting the sheer scale of the challenge of creating a machine that could understand and compose music like a human can.
The data came from combining information from three large-scale public sources, two of them originally intended for entirely different purposes. The authors downloaded the lyrics and chord sequences of nearly 90,000 popular songs from Ultimate Guitar, a longstanding community website where users upload their own transcriptions of music.
To match the lyrics of the songs to emotions, the researchers took data from labMT, a crowd-sourced website that rates the emotional valence of words (the degree to which they represent good or bad feelings). The details of when and where the songs originated from were taken from Gracenote, the same database as your music player probably uses to show artists’ information.
By correlating the valence of words with the type of chord accompanying them, the authors confirmed that major chords were associated more with positive words than minor chords. Unexpectedly, they found that seventh chords—chords with four different notes rather than the usual three—had an even higher association with positive words, even in the case of minor seventh chords. This is in constrast to other studies which have placed the valence of seventh chords between minor and major.
More here.