Neural Correlates of Gratitude (2015)
An experiment was conducted during which gratitude was induced in participants while they underwent functional magnetic resonance imaging. The hypothesis was that gratitude ratings would correlate with activity in brain regions associated with moral cognition, value judgment and theory of mind. The stimuli used to elicit gratitude were drawn from stories of survivors of the Holocaust, as many survivors report being sheltered by strangers or receiving lifesaving food and clothing, and having strong feelings of gratitude for such gifts.

The Role of Gratitude in Spiritual Well-Being in Asymptomatic Heart Failure Patients (2015)
This study examined associations between gratitude, spiritual well-being, sleep, mood, fatigue, cardiac-specific self efficacy, and inflammation in 186 men and women with Stage B asymptomatic heart failure.
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You Didn’t Have to Do That: Belief in Free Will Promotes Gratitude (2014)
Four studies tested the hypothesis that a weaker belief in free will would be related to feeling less gratitude.

The Big Benefits of a Little Thanks (2013)
Francesca Gino and Adam Grant, of Harvard Business School and Wharton, respectively, discuss their research on gratitude and how even a tiny amount of gratitude can have a huge impact.

Graduate counseling psychology students’ experiences of mindfulness meditation and gratitude journaling (2013)
Using a qualitative design with thematic analysis, a 15-minute mindfulness meditation and gratitude journalling intervention was conducted with 9 graduate counseling psychology students.

Find, Remind, and Bind: The Functions of Gratitude in Everyday Relationships (2012)
Within the context of reciprocally-altruistic relationships, gratitude signals communal relationship norms and may be an evolved mechanism to fuel upward spirals of mutually responsive behaviors between recipient and benefactor.

At the 2014 Greater Good Gratitude Summit, Sara B. Algoe of University of North Carolina, Chapel Hill, describes her research into how gratitude affects romantic partners’ feelings for one another, as well as their style of relating to each other.

To Have and to Hold: Gratitude Promotes Relationship Maintenance in Intimate Bonds (2012)
This multi-method series of studies merges the literatures on gratitude and risk regulation to test a new process model of gratitude and relationship maintenance.
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Effects of Constructive Worry, Imagery Distraction, and Gratitude Interventions on Sleep Quality (2011)
E-mailed self-help versions of constructive worry, imagery distraction, or a gratitude intervention helped university students quiet their minds and sleep better.

Death and gratitude: Death reflection enhances gratitude (2011)
Participants were randomly assigned to a death reflection condition, a traditional mortality salience condition, or to a control condition. Participants in the death reflection and the mortality salience conditions showed enhanced gratitude compared to individuals in the control condition, supporting the theory that becoming aware of one’s mortal limitations enhances gratitude for the life that what one has.
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Measuring Gratitude in Youth: Assessing the Psychometric Properties of Adult Gratitude Scales in Children and Adolescents (2011)
This study is an empirical investigation, based on a large youth sample with ages ranging from 10 to 19 years old, of the psychometric properties of scores of the Gratitude Questionnaire-6, the Gratitude Adjective Checklist, and the Gratitude Resentment and Appreciation Test.
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A Grateful Heart is a Nonviolent Heart: Cross-Sectional, Experience Sampling, Longitudinal, and Experimental Evidence (2011)
Five studies tested the hypothesis that gratitude is linked to lower levels of aggression.

It’s the Little Things: Everyday Gratitude as a Booster Shot for Romantic Relationships (2010)
Drawing on a social functional model of emotions, this study tested the roles of gratitude and indebtedness in romantic relationships with a daily-experience sampling of both members of cohabiting couples.
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Gratitude and Well-Being: A Review and Theoretical Integration (2010)
This paper presents a new model of gratitude incorporating not only the gratitude that arises following help from others but also a habitual focusing on and appreciating the positive aspects of life.
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Who Benefits The Most from a Gratitude Intervention in Children and Adolescents? Examining Positive Affect as a Moderator (2009)
This study examined if positive affect moderated the effects of a gratitude intervention where youth were instructed to write a letter to someone whom they were grateful and deliver it to them in person.
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Gratitude influences sleep through the mechanism of pre-sleep cognitions (2009)
This study tested whether individual differences in gratitude are related to sleep after controlling for neuroticism and other traits.
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Beyond Reciprocity: Gratitude and Relationships in Everyday Life (2008)
The authors examined the role of naturally occurring gratitude in college sororities during a week of gift-giving from older members to new members.

An Adaptation for Altruism? The Social Causes, Social Effects, and Social Evolution of Gratitude (2008)
Evolutionary theories propose that gratitude is an adaptation for reciprocal altruism (the sequential exchange of costly benefits between nonrelatives) and, perhaps, upstream reciprocity (a pay-it-forward style distribution of an unearned benefit to a third party after one has received a benefit from another benefactor). Gratitude therefore may have played a unique role in human social evolution.
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A social-cognitive model of trait and state levels of gratitude (2008)
Three studies tested a new model of gratitude, which specified the generative mechanisms linking individual differences (trait gratitude) and objective situations with the amount of gratitude people experience after receiving aid (state gratitude).
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Gratitude and prosocial behavior: helping when it costs you (2006)
The ability of the emotion gratitude to shape costly prosocial behavior was examined in three studies employing interpersonal emotion inductions and requests for assistance.
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How to Increase and Sustain Positive Emotion: The Effects of Expressing Gratitude and Visualizing Best Possible Selves (2006)
A 4-week experimental study examined the motivational predictors and positive emotion outcomes of regularly practicing two mental exercises: counting one’s blessings (“gratitude”) and visualizing best possible selves (“BPS”).
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Counting Blessings Versus Burdens: An Experimental Investigation of Gratitude and Subjective Well-Being in Daily Life (2003)
The effect of a grateful outlook on psychological and physical well-being was examined.
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Gratitude and Happiness: Development of a Measure of Gratitude and Relationships with Subjective Well-Being (2003)
Four studies were conducted evaluating the reliability and validity of the Gratitude Resentment and Appreciation Test (GRAT), a measure of dispositional gratitude.
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The Grateful Disposition: A Conceptual and Empirical Topography (2002)
In four studies, the authors examined the correlates of the disposition toward gratitude.
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Thanks to the Greater Good Science Center for posting PDFs of studies on their website.