The act of writing a thank-you note after receiving a gift can be a knee-jerk response for some, groomed from childhood to always say "Thank you" even when the person isn't present. Others find themselves procrastinating the gesture, relegating it for tomorrow, then the next week, until the moment passes and eventually all that surfaces is a consolatory remembrance many months down the line.

But writing thank-you notes carries a more selfish incentive, studies find. When people list the things they're grateful for, their overall mood improves and their depressive symptoms subside. You feel more at ease, less stressed, and more mindful of the things that make you happy — which, after writing the thank-you notes, can itself be added to the list.

The Study

In 2012, researchers at Kent State University conducted tests on 219 individuals, 188 of whom were women, building on prior studies performed in 2009 by doctors Steven Toepfer and Kathleen Walker, both associate professors in Human Development and Family Studies. The new team, still led by Toepfer, divided their participants into two groups, writers and non-writers.

The pair had the subjects complete questionnaires once a week for four weeks. During weeks two, three, and four the subjects wrote a letter of gratitude. The letters ranged from half a page to a full page in length, and participants both hand wrote and typed the letters. Toepfer and his team did not believe the method of letter-writing affected the outcome.

During the measurement periods, the team looked at four distinct levels: gratitude, life satisfaction, happiness, and depressive symptoms.

According to the definitions in their 2009 study, the first three measurements contained the following criteria:

Gratitude: typically comprised of appreciation, thankfulness, and a sense of wonder... It indicates that people can extract the most satisfaction and enjoyment from life events and facilitates positive experiences.

Life Satisfaction: commonly referred to as the cognitive and personal assessment of general quality of life and is based on unique or personalized criteria that varies among individuals.

Happiness: a feeling of gladness and satisfaction or contentment, suggesting increased insight, and therefore subjective selection and consideration about the important things in one's life.

The newest trait, depressive symptoms, was included only as part of the 2012 study, where it was defined as such:

Depressive Symptoms: assessments of a negative affect and its contribution to well-being. Depression can be defined not only by high levels of negative affect but also relative levels compared to positive affect.

The Results

The results of the studies bolstered Toepfer's and Walker's earlier findings in 2009, the team said. Not only did the larger sample size agree with the increases in happiness, life satisfaction, and gratitude, but the new component of depressive symptoms decreased substantially.

"The implications are that this type of expressive writing can benefit those who suffer from depressive symptoms," concluded the researchers. "Further research is necessary, but gratitude letters may be a simple intervention for those who struggle with such symptomatology."

While the researchers acknowledge that attaching a definition to subjective, nebulous concepts like happiness can compromise the study, their methodology has salience.

Founder of behavioral economics and Nobel Prize-winning economist, Daniel Kahneman claims happiness is two-faced. It's split, in other words, between an experiencing self and a remembering self. The experiencing self lives in the moment, while the remembering self stores that moment to look back on while experiencing later moments.

Writing thank-you notes seems to serve both selves, perhaps explaining why they reinforce such positive emotions in the writer. His or her experiencing self finds immense gratitude in the writing process itself, of transferring heartwarming ideas to the page.

All the while, it's drawing upon the remembering self in order to formulate those ideas. They're where the ideas come from. You're happy presently showing gratitude, and you're happy reliving the moments you're writing about.

"We know that happiness is mainly being satisfied with people that we like, spending time with people that we like," said Kahnmen. "There are other pleasures, but this is dominant."