With the graphs spread out in front of him, Bentley says, the patterns are easy to see. “The ’20s were the highest peak of joy-related words that we see,” he says. “They really were roaring.”

But then came 1941, which, of course, marked the beginning of America’s entry into World War II. It doesn’t take a historian to see that peaks and valleys like these roughly mirror the major economic and social events of the century.

– Alix Spiegel in Using Books to Map Emotions Through a Century

A fasinating and strange analysis of the text of billions of books.