Harmonic surprise - an unexpected change in music - as well as the use of relatively rare chords trigger pleasurable response in the listener's brain, say scientists who have decoded what makes certain songs top the charts.
Researchers from the Georgetown University in the US uncovered a simple, measurable explanation that can determine people's preference for one song over another.
The study, published in the journal Frontiers in Human Neuroscience, linked the harmonic structure of pop songs to their placement in the charts.
"The most popular songs tend to include relatively rare chords, that is, they typically have high harmonic surprise," said Norberto Grzywacz, a professor at Georgetown University.
"These songs also tend to have choruses with relatively low harmonic surprise preceded by sections with many rare chords," said Grzywacz.
Harmonic surprise can be described as where the music deviates from the listeners expectations. Scientists have predicted that these changes in structure could elicit a pleasurable reward response in the brain.
In other words, harmonic surprise can increase the likelihood a song will be a hit.
"When listening to music, we enjoy some pieces and dislike others. Multiple reasons govern how much we like a piece of music, including compositional, emotional and cultural," said Grzywacz.
"We evaluated the role of a compositional element - the harmonic surprise. Surprise is important because it is a measure of new information; something that the reward centres of the brain recognise as being of value, leading to a positive emotional response," he said.
"Therefore, our finding that the most popular songs tend to include surprising chords reflects our brains in-built preference," Grzywacz said.
It is not just the surprise element of a song that the brain deems as pleasurable, but the return to normality too.
"The brain enjoys surprise only up to a point, because unexpected events indicate a failure of prediction," he said.
"Hence, the release of tension from surprising sections of a song to common choruses is also signalled positively by the reward centres," he said.
"Our research reveals that the brain has a deep-rooted preference, which can affect whether people enjoy a piece of music," he added.
The study analysed chord-by-chord transcriptions of the harmonies of 545 songs that entered the American Billboard Hot 100 charts between 1958 and 1991.
Researchers measured how far chords of a song deviated from what was expected. These measures of surprise were compared throughout the entire song and between song sections.
"We then used the peak position of the song in the weekly Billboard Hot 100 chart to determine its popularity," said Grzywacz.
It revealed that verses, not the choruses or bridges, accounted for much of the difference in harmonic surprise between the most and least popular songs in the Billboard Hot 100 chart.
Researchers suggest that high surprises in the harmony of a song, as well as high surprises followed by a lower-surprise section, can both contribute to the enjoyment of an unfamiliar piece of music.
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