Why do we help our friends, family, and co-workers at a cost to our time or money? One might answer, “Because they helped me out in the past,” or, “Because they will help me out in the future.” Such explanations invoke the notion of reciprocity; if I help you out today, you are more likely to help me out tomorrow. However, we also often help strangers who we will never meet again, or we help animals who can provide no...