There has been some debate on whether or not humans are truly capable of psychological altruism. Some definitions specify a self-sacrificial nature to altruism and a lack of external rewards for altruistic behaviors. However, because altruism ultimately benefits the self in many cases, the selflessness of altruistic acts is brought to question. The social exchange theory postulates that altruism only exists when benefits to the self outweigh costs to the self. Daniel Batson is a psychologist who examined this question and argues against the social exchange theory. He identified four major motives: to ultimately benefit the self (egoism), to ultimately benefit the other person (altruism), to benefit a group (collectivism), or to uphold a moral principle (principlism). Altruism that ultimately serves selfish gains is thus differentiated from selfless altruism, but the general conclusion has been that empathy-induced altruism can be genuinely selfless. The "empathy-altruism hypothesis" basically states that psychological altruism does exist and is evoked by the empathic desire to help someone who is suffering. Feelings of empathic concern are contrasted with feelings of personal distress, which compel people to reduce their own unpleasant emotions. People with empathic concern help others in distress even when exposure to the situation could be easily avoided, whereas those lacking in empathic concern avoid helping unless it is difficult or impossible to avoid exposure to another's suffering. Helping behavior is seen in humans at about two years old, when a toddler is capable of understanding subtle emotional cues.