Whether you love or hate Twitter, the microblogging site has revolutionized online communication and some scientists are even using it to ascertain worldwide mood. Though, what they’ve found is kind of a bummer.

In particular, Twitter has seen a steady decline in happy words and an increase in negative words since 2009, with the steepest decline beginning in early 2011 with no sign of improvement.

In a recent study, researchers at the University of Vermont assigned a happiness score of 1-9 to the 10,000 most common words in the English language. For example, “laughter” was given a relatively high average happiness score of 8.50 and “food” a 7.44, while “greed” and “terrorist” scored much lower, with scores of 3.06 and 1.30 respectively.

Afterward, those words were tracked in 4.6 billion tweets posted by more than 63 million Twitter users during a three-year period to ascertain people’s overall mood. According to the results, it seems societal happiness is on the decline.

As you might expect, the death of Osama bin Laden — when words like “death” and “killed” were common — produced the most unhappy words used on Twitter during the entire study, while the happiest days were weekends and holidays.

Outside of those, the happiest non-annual day during the study was April 29, 2011, the day Prince William and Kate Middleton tied the knot and words like “wedding,” “beautiful” and “kiss” abounded.

But you may want to take sometimes mercurial Twitter moods with a grain of salt. The study is careful to say, “There is an important psychological distinction between an individual’s current, experiential happiness and their longer term, reflective evaluation of their life.”

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