" Sister Cecilia used the words “very happy” and “eager joy,” both expressions of effervescent good cheer. Sister Marguerite’s autobiography, in contrast, contained not even a whisper of positive emotion. When the amount of positive feeling was quantified by raters who did not know how long the nuns lived, it was discovered that 90 percent of the most cheerful quarter was alive at age eighty-five versus only 34 percent of the least cheerful quarter. Similarly, 54 percent of the most cheerful quarter was alive at age ninety-four, as opposed to 11 percent of the least cheerful quarter. Was it really the upbeat nature of their sketches that made the difference? Perhaps it was a difference in the degree of unhappiness expressed, or in how much they looked forward to the future, or how devout they were, or how intellectually complex the essays were. But research showed that none of these factors made a difference, only the amount of positive feeling expressed in the sketch. So it seems that a happy nun is a long-lived nun. "

― Martin E.P. Seligman , Authentic Happiness: Using the New Positive Psychology to Realize Your Potential for Lasting Fulfillment