The researchers made some remarkable discoveries about the evolution of word usage in English books over the past century. Firstly, 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’).

Dr Acerbi said: “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.”
This suggests that the divergence in emotional content between the two forms of English is paired by a more general stylistic divergence.

Fear factor increases, emotions decrease in books written in last 50 years (via myserendipities)