Abstract
Gratitude may function to foster human sociality. Exactly how gratitude performs this putative function, however, is a question that has received little scientific attention. Many recent investigations—primarily concerned with the positive emotional outcomes associated with the experience of gratitude—have overlooked the fundamental mechanistic operations that might produce gratitude. My dissertation addresses this oversight by capitalizing on advancements in the evolutionary psychological study of function and internal regulatory variables. I approach gratitude as a psychological adaptation that functions to coordinate behaviors that initiate and strengthen interpersonal relationships in conjunction with an internal regulatory variable responsible for tracking interpersonal welfare valuation, the Welfare Tradeoff Ratio (WTR). I conducted three studies that induced gratitude between strangers (Study One), friends (Study Two), and siblings (Study Three), and assessed for changes in WTR. Positive changes in WTRs for strangers coincided with an emotional response of gratitude and a series of affiliative behaviors that presumably act to initiate new relationships. Null changes, as opposed to positive or negative changes, in the already high WTRs of friends and siblings for each other, also coincided with gratitude and behaviors that presumably act to strengthen established bonds.