The first chapter, The Girl Scout Cookie Phenomenon, uses a laboratory experiment to study favor trading in a public goods setting. The ability to practice targeted reciprocity increases contributions by 14%, which corresponds directly to increased efficiency. Subjects discriminate by rewarding group members who have been generous and withholding rewards from ungenerous group members. At least some reciprocal behavior is rooted in other-regarding preferences. When someone is outside the circle of reciprocity, he gives less to the public good than in other settings. We find no evidence of indirect reciprocity. We find two behavioral types in each treatment, differing in baseline giving but not in tendency to reciprocate.