Researchers from the Universities of Bristol, Sheffield, and Durham also found a divergence between American and British English, with the former being 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 were initially surprised to see how well periods of positive and negative moods correlated with historical events.
More From This Section
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.
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.