After psychologist Martin Seligman started teaching what he soon came to call "positive psychology," he began to see some remarkable results.  As he puts it, "I do not think I ever heard the word 'life-changing' more often in my 40 years of teaching."

Two out of the three most effective happiness-inducing exercises he tested were shown to be related to the practice of gratitude. In a carefully-controlled scientific study, he found:

"Three exercises proved to be placebos and three worked well, producing lasting reductions in depression and lasting increases in happiness.

The three that worked were 'three blessings' - writing down three things that went well today and why; 'the gratitude visit' - writing a gratitude testimonial and delivering it personally; and 'using your signature strength in a new way' - taking the signature strength test and using your highest strength in a new way."

Of the effect of the first exercise, he writes:

"We looked at the effect on severe depression of doing just the first web exercise, three blessings.

Depressive symptoms were substantially reduced and happiness markedly increased.
 
In this uncontrolled study, 94% of severely depressed people became less depressed and 92% became happier, with an average symptom relief of a whopping 50% over only 15 days.

This compares very favourably with anti-depressant medication and with psychotherapy."

- Professor Martin Seligman, Director, Positive Psychology Center, University of Pennsylvania, "Staying Happier for Longer," BBC News, May 9, 2006