Two psychologists, Dr. Robert A. Emmons and Dr. Michael E. McCullough, have done much of the research on gratitude. In one study, they asked all participants to write a few sentences each week, focusing on particular topics. One group wrote about things they were grateful for that had occurred during the week. A second group wrote about daily irritations or things that had displeased them, and the third wrote about events that had affected them (with no emphasis on them being positive or negative). After 10 weeks, those who wrote about gratitude were more optimistic and felt better about their lives. Surprisingly, they also exercised more and had fewer visits to physicians than those who focused on sources of aggravation.

Another leading researcher in this field, Dr. Martin E. P. Seligman, tested the impact of various assignments given to help the mood of 411 people. When their assignment was to write and personally deliver a letter of gratitude to someone who had never been properly thanked for his or her kindness, participants immediately exhibited a huge increase in happiness scores. This impact was greater than that from any other assignment, with benefits lasting for a month.

Of course, studies such as this one cannot prove cause and effect. But most of the studies published on this topic support an association between gratitude and an individual's well-being.