The study of forgiveness within psychology and other social sciences has grown admirably over the past 25 years, but scientific understanding of forgiveness is not well linked to a highly relevant body of ideas and discoveries from evolutionary biology and evolutionary psychology. In addition, the field has been criticized (largely from within) for its nearly exclusive reliance on self-report measures to assess forgiveness, which greatly limits confidence about both the causes of forgiveness, the computational processes underlying forgiveness decisions, and the social effects of forgiveness. In this collaborative project, which brings together the resources and talents of five different investigators in the United States and Japan, we propose a research program that uses an evolutionary-functional approach to better understand the mechanisms that regulate humans’ decisions about when to forgive and how to respond to gestures of forgiveness from others. In a series of 14 experiments with university students and community-dwelling adults, which involve new and existing experimental methods from behavioral economics and social psychology, we will seek to identify the cues that the human mind has evolved to process as a way of adaptively regulating decisions about when to forgive and how to respond when one has been forgiven. The members of our research team bring unique strengths from social psychology, evolutionary psychology, and behavioral economics. By (a) publishing research findings in the leading journals within social psychology, experimental economics, and (hopefully) general science journals; (b) introducing new behavioral economics and social psychology methods for studying forgiveness; and (c) speaking publicly and writing about our work, we hope to shed unique scientific light on--and stimulate more evolutionarily informed thinking about--this vital human trait.