Altruism in simple term is self-sacrifice, a behavior that increases another person's welfare and not oneself. The theory is based on egoism, that the genes are selfish thus humans tend to favor kin because of similarity in genes. Economists assume that this behavior is mainly driven by norms and fairness. Social psychologists, however, consider empathy to be a key motivator to understanding altruistic behavior. Empathy is the ability to understand and share the feelings of another. A study on human behavior had shown that the increase in experienced empathy predicted over 40% of the increase in sharing or altruist behavior.