American books became more emotional than British ones starting in the 1960s, a new study claimed Monday.

The study, published in the journal PLOS One, also found that certain “mood words” were used more than others in various periods of the 20th century.

“We were initially surprised to see how well periods of positive and negative moods correlated with historical events,” Dr. Alberto Acerbi, one of the researchers, said in a statement.

“The second world war, for example, is marked by a distinct increase in words related to sadness, and a correspondent decrease in words related to joy.”

The researchers used Ngram, a database managed by Google, to search for certain words, according to MinnPost.

They found an overall decrease in the use of mood words across the English-language books archived on Ngram.

American authors, the researchers noted, began to use more emotional words than British ones in the 1960s.

“In the USA, baby boomers grew up in the greatest period of economic prosperity of the century,” said co-author Alex Bentley.

“Whereas the British baby boomers grew up in a post-war recovery period, so perhaps 'emotionalism' was a luxury of economic growth."