Algoe, S. B., Haidt, J., &Gable, S. L. (2008). Beyond reciprocity: Gratitude and relationships in everyday life. Emotion, 8(3), 425-429.

The emotion of gratitude is thought to have social effects, but empirical studies of such effects have focused largely on the repaying of kind gestures. The current research focused on the relational antecedents of gratitude and its implications for relationship formation. The authors examined the role of naturally occurring gratitude in college sororities during a week of gift-giving from older members to new members. New members recorded reactions to benefits received during the week. At the end of the week and 1 month later, the new and old members rated their interactions and their relationships. Perceptions of benefactor responsiveness predicted gratitude for benefits, and gratitude during the week predicted future relationship outcomes. Gratitude may function to promote relationship formation and maintenance.

http://dx.doi.org/10.1037/1528-3542.8.3.425