more 'emotional' than the latter.
The researchers looked at how frequently 'mood' words were used through time in a database of more than five million digitised books provided by Google.
The list of words was divided into six categories (anger, disgust, fear, joy, sadness, surprise) previously used by one of the researchers, Dr Vasileios Lampos, to detect contemporary mood changes in public opinion as expressed in tweets collected in the UK over more than two years.
"We thought that it would be interesting to apply the same methodology to different media and, especially, on a larger time scale," said Dr Alberto Acerbi, a Newton Fellow in the Department of Archaeology and Anthropology at the University of Bristol.
"We were initially surprised to see how well periods of positive and negative moods correlated with historical events.
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," said Acerbi, lead author of the paper published in journal PLOS ONE.
In applying this technique, the researchers found the emotional content of published English has been steadily decreasing over the past century, with the exception of words associated with fear, an emotion which has resurged over the past decades.
They also found that American English and British English have undergone a distinct stylistic divergence since the 1960s. American English has become decidedly more 'emotional' than British English in the last half-century.
The same divergence was also found in the use of content-free words, that is words which carry little or no meaning on their own, such as conjunctions ('and', 'but') and articles ('the').
"This is particularly fascinating because it has recently been shown that differences in usage of content-free words are a signature of different stylistic periods in the history of western literature," Acerbi said.
"We don't know exactly what happened in the Sixties but our results show that this is the precise moment in which literary American and British English started to diverge. We can only speculate whether this was connected, for example, to the baby-boom or to the rising of counterculture," co-author Professor Alex Bentley said.
"In the USA, baby boomers grew up in the greatest period of economic prosperity of the century, whereas the British baby boomers grew up in a post-war recovery period so perhaps 'emotionalism' was a luxury of economic growth," Bentley said.