Researchers used Google’s Ngram program to evaluate the language used in books, and found that fear-based words are on the rise while other emotional expression is on the decline.

Writers agonize about finding just the right words to express their emotions, observations, and musings.

“Words mean more than what is set down on paper,” poet and civil rights activist Maya Angelou wrote in I Know Why the Caged Bird Sings. “It takes the human voice to infuse them with shades of deeper meaning.”

Since the 1970s, fear language appears more often in literary works than language pertaining to any other emotion, and books written in the second half of the 20th century contain less emotional language overall than those written in the first half, according to a new study.

Researchers from the University of Bristol in the U.K. used Google’s Ngram database to analyze the type and prevalence of emotional language in books written during the 20th century. Their findings were published Wednesday in the journal PLoS One.

They examined the use of words that convey anger, disgust, fear, joy, sadness, and surprise in nearly 5.2 million books published in English between 1900 and 2000.

Researchers found that the words writers chose changed to reflect important social and political events of the time. For example, “happy” language peaked at the end of WWI, while “sad” language peaked during WWII.

Differentiating between American and British English, researchers noted that American authors have typically been more “emotional” than British writers in the past 50 years.

Currently—or at least up to the year 2000—we appear to be living in a literary landscape of fear. The data show that the use of other forms of emotional language has been rapidly disappearing from both fiction and non-fiction books.

While other studies have focused on the use of language in social media to reflect the social and political climate, the University of Bristol study is the first to examine how moods expressed in books can reflect long-term historical and cultural trends.

Researchers noted that one large, outstanding question is whether word usage reflects the real behavior of people at the time of writing or whether literature instead achieves its ultimate goal: allowing readers to live out their fantasies of how society ought to be.

“It has been suggested, for example, that it was the suppression of desire in ordinary Elizabethan English life that increased demand for writing ‘obsessed with romance and sex,’” the study authors wrote. “So while it is easy to conclude that Americans have themselves become more ‘emotional’ over the past several decades, perhaps songs and books may not reflect the real population any more than catwalk models reflect the average body.”

Still, the researchers said they believe the changes in language they identified reflect culture changes, as all the books they analyzed were given the same weight, without commercial value or sales figures taken into consideration.

It goes without saying that the language of literature can be used with great effect to describe the pleasure of prosperous times and the volatile nature of war.

As Aldous Huxley wrote in Brave New World, “Words can be like X-rays if you use them properly—they’ll go through anything. You read and you’re pierced.”

The beginning of the 1920s was a happy time for many, and, not coincidentally, that’s when “happy” language peaked in American books.

The decade began on the heels of the end of WWI. It was also when women were given the right to vote, movies featured sound for the first time, and Mickey Mouse was born, so there were many happy events worth writing about. It was also the beginning of Prohibition, however, so there was plenty of sad stuff to mention as well.

Then, just like the economy of the Great Depression, the stock of positive language plummeted from an all time high in the 1920s to record lows leading up to WWII.

On a much lesser scale, happiness and joy in literature rose again from the ashes of WWII, continued through the prosperous 1950s, and hit a plateau during the 1960s. This was when literature broke through long-held social boundaries, particularly with the Beat Generation’s exploration of drugs, sexuality, and themes of spiritual freedom.

It was also the dawn of New Journalism in America, when writers like Tom Wolfe, Norman Mailer, Hunter S. Thompson, and Joan Didion used new techniques to document the landscape of political unrest, empowerment, and the search for freedom during the Civil Rights movement, the Cuban missile crisis, the space race, the assassination of Pres. John F. Kennedy, and the start of the Vietnam War.

During the 1970s and 1980s, “sad” language was on the rise, and this trend continued on until the 1990s when happiness and other emotions crept back into books.

At the same time, however, the use of fear-based language began its upward surge.

Words conveying fear declined through the earliest decades of the 20th century, but have been on a steady climb since 1970. Words relaying other emotions have been in declining use, even in U.S. literature.

There are numerous possible reasons why this happened, but one shouldn’t be ignored: the emergence of commerically-successful horror movies, beginning with such films as Rosemary’s Baby, Night of the Living Dead, and The Exorcist. These films helped change the language of society by offering us many more things to be scared about besides Mary Shelley’s Frankenstein.

The researchers’ data ended at the year 2000, right before the U.S. entered a whole new era of fear: the September 11th attacks, the Patriot Act, an economic crisis, and wars in Iraq and Afghanistan.

It’s not a stretch to assume that the rise in fear-based language researchers observed has continued unabated in post-9/11 America.