ScienceDaily (Dec. 16, 2011) — The gross domestic product of the United States -- that oft-cited measure of economic health -- has been ticking upward for the last two years.

But what would you see if you could see a graph of gross domestic happiness?

A team of scientists from the University of Vermont has made such a graph -- and the trend is down.

Reporting in the Dec. 7 issue of the journal PLoS ONE, the team writes, "After a gradual upward trend that ran from January to April, 2009, the overall time series has shown a gradual downward trend, accelerating somewhat over the first half of 2011."

"It appears that happiness is going down," said Peter Dodds, an applied mathematician at UVM and the lead author on the new study.

How does he know this? From Twitter. For three years, he and his colleagues gathered more than 46 billion words written in Twitter tweets by 63 million Twitter users around the globe.

In these billions of words is not a view of any individual's state of mind. Instead, like billions of moving atoms add up to the overall temperature of a room, billions of words used to express what people are feeling resolve into a view of the relative mood of large groups.