Altruism is acting out of concern for another’s well-being. Often, people behave altruistically when they see others in desperate circumstances and feel empathy and a desire to help. Altruism doesn’t always come naturally, since by definition, it requires people to disregard their own concerns to help others without any expectation of reward, though "reciprocal altruism" is a term used by evolutionary biologists and psychologists to characterize the decision to help with an expectation that one will receive some benefit or pay-off to oneself. Cooperative behavior allowed our ancestors to survive under harsh conditions, and it still serves a necessary purpose to a highly complex society. Even when people don’t behave altruistically for recognition or reward, they often feel energized and happy after helping others, sometimes called the 'helper's high." Humans aren’t the only animals who behave altruistically. Many species benefit when individual organisms disregard personal costs and act in service of the larger group.