The authors analyzed trends in the use of 'mood words' that convey joy, anger, fear and other emotions and found a general, overall decrease in the use of words that convey emotions. They found distinct historical periods of positive and negative moods in books that correlated with socio-political events like World War II, the Great Depression or the Baby Boom.
In contrast to the five other moods studied, words that convey fear showed an opposite trend. Words conveying fear declined in use through the early 20th century but increased markedly since the 1970s, despite the decreased use of other 'mood words'. Lastly, the authors found that books in American English are significantly more 'emotional' than British English books written after 1960.
Previous studies have shown that words used in online social media can reflect recent socio-political events, but this is the first study to reveal that the expression of moods in books can also reflect longer-term historical or cultural trends.
Source: Public Library of Science