Pop songs have become angrier and sadder over the past 60 years, experts say.
Researchers analyzed lyrics in best-selling songs from the 1950s to 2016 to find expressions of anger and sadness had increased, while words about joy had dropped.
The US study team looked at lyrics of more than 6,000 songs from Billboard Hot 100 in each year.
These are the most popular songs in the US each year as chosen by music fans.
In the past songs were ranked mainly by record sales, radio and jukebox plays, but more recently it is based on other popularity indicators such as streaming and social media to reflect changes in music consumption.
Tones expressed in each song were analyzed using 'automatic quantitative sentiment' which looked at each word or phrase in the song with a set of tones they express.
The combination of the tones expressed by all words and phrases of the lyrics determines the sentiment of that song.
The sentiments of all Billboard Hot 100 songs in each year are averaged and the average of each year measured whether the expression of that sentiment increased, decreased or remained the same.
The analysis showed the expression of anger in popular music lyrics has increased gradually over time.
Study co-author Lior Shamir, of Lawrence Technological University in Michigan, said: 'The change in lyrics sentiments does not necessarily reflect what the musicians and songwriters wanted to express, but is more related to what music consumers wanted to listen to in each year.'
Songs released during the mid 1950s were the least angry and the anger expressed in lyrics has increased gradually until peaking in 2015.
In the mid 1990s, songs became angrier and the increase in anger was sharper during that time in comparison to previous years.
The expression of sadness, disgust and fear also increased over time, although the increase was milder compared to the increase in the expression of anger.
Disgust increased gradually, but was lower in the early 1980s and higher in the mid and late 1990s.
Popular music lyrics expressed more fear during the mid 1980s and the fear decreased sharply in 1988.
Another sharp increase in fear was observed in 1998 and 1999, with a sharp decrease in 2000.
The study also showed that joy was a dominant tone in popular music lyrics during the late 1950s, but it decreased over time and became much milder in the recent years.
An exception was observed in the mid 1970s, when joy expressed in lyrics increased sharply.
The study shows that the tones expressed in popular music change over time and the change is gradual and consistent, with a few exceptions.
Since the researchers analyzed the most popular songs in each year, the study does not show that music changed, but that the preferences of music consumers have changed over time.
While music fans preferred joyful songs during the 1950s, modern music consumers are more interested in songs that express sadness or anger.
The findings appear in the Journal of Popular Music Studies.