Psychological egoism is the view that humans are always motivated by self-interest and selfishness, even in what seem to be acts of altruism. It claims that, when people choose to help others, they do so ultimately because of the personal benefits that they themselves expect to obtain, directly or indirectly, from so doing. This is a descriptive rather than normative view, since it only makes claims about how things are, not how they ought to be. It is, however, related to several other normative forms of egoism, such as ethical egoism and rational egoism. Psychological egoism is the view that humans are always motivated by self-interest and selfishness, even in what seem to be acts of altruism. It claims that, when people choose to help others, they do so ultimately because of the personal benefits that they themselves expect to obtain, directly or indirectly, from so doing. This is a descriptive rather than normative view, since it only makes claims about how things are, not how they ought to be. It is, however, related to several other normative forms of egoism, such as ethical egoism and rational egoism. A specific form of psychological egoism is psychological hedonism, the view that the ultimate motive for all voluntary human action is the desire to experience pleasure or to avoid pain. Many discussions of psychological egoism focus on this type, but the two are not the same: theorists have explained behavior motivated by self-interest without using pleasure and pain as the final causes of behavior. Psychological hedonism argues actions are caused by both a need for pleasure immediately and in the future. However, immediate gratification can be sacrificed for a chance of greater, future pleasure. Further, humans are not motivated to strictly avoid pain and only pursue pleasure, but, instead, humans will endure pain to achieve the greatest net pleasure. Accordingly, all actions are tools for increasing pleasure or decreasing pain, even those defined as altruistic and those that do not cause an immediate change in satisfaction levels. Beginning with ancient philosophy, Epicureanism claims humans live to maximize pleasure. Epicurus argued the theory of human behavior being motivated by pleasure alone is evidenced from infancy to adulthood. Humanity performs altruistic, honorable, and virtuous acts not for the sake of another or because of a moral code but rather to increase the well being of the self. In modern philosophy, Jeremy Bentham asserted, like Epicurus, that human behavior is governed by a need to increase pleasure and decrease pain. Bentham explicitly described what types and qualities of pain and pleasure exist, and how human motives are singularly explained using psychological hedonism. Bentham attempted to quantify psychological hedonism. Bentham endeavored to find the ideal human behavior based on hedonic calculus or the measurement of relative gains and losses in pain and pleasure to determine the most pleasurable action a human could choose in a situation. From an evolutionary perspective, Herbert Spencer, a psychological egoist, argued that all animals primarily seek to survive and protect their lineage. Essentially, the need for the individual and for the individual's immediate family to live supersedes the others' need to live. All species attempt to maximize their own chances of survival and, therefore, well being. Spencer asserted the best adapted creatures will have their pleasure levels outweigh their pain levels in their environments. Thus, pleasure meant an animal was fulfilling its egoist goal of self survival, and pleasure would always be pursued because species constantly strive for survival. Whether or not Sigmund Freud was a psychological egoist, his concept of the pleasure principle borrowed much from psychological egoism and psychological hedonism in particular. The pleasure principle rules the behavior of the Id which is an unconscious force driving humans to release tension from unfulfilled desires. When Freud introduced Thanatos and its opposing force, Eros, the pleasure principle emanating from psychological hedonism became aligned with the Eros, which drives a person to satiate sexual and reproductive desires. Alternatively, Thanatos seeks the cessation of pain through death and the end of the pursuit of pleasure: thus a hedonism rules Thanatos, but it centers on the complete avoidance of pain rather than psychological hedonist function which pursues pleasure and avoids pain. Therefore, Freud believed in qualitatively different hedonisms where the total avoidance of pain hedonism and the achievement of the greatest net pleasure hedonism are separate and associated with distinct functions and drives of the human psyche. Although Eros and Thanatos are ruled by qualitatively different types of hedonism, Eros remains under the rule of Jeremy Bentham's quantitative psychological hedonism because Eros seeks the greatest net pleasure. Traditional behaviorism dictates all human behavior is explained by classical conditioning and operant conditioning. Operant conditioning works through reinforcement and punishment which adds or removes pleasure and pain to manipulate behavior. Using pleasure and pain to control behavior means behaviorists assumed the principles of psychological hedonism could be applied to predicting human behavior. For example, Thorndike's law of effect states that behaviors associated with pleasantness will be learned and those associated with pain will be extinguished. Often, behaviorist experiments using humans and animals are built around the assumption that subjects will pursue pleasure and avoid pain. Although psychological hedonism is incorporated into the fundamental principles and experimental designs of behaviorism, behaviorism itself explains and interprets only observable behavior and therefore does not theorize about the ultimate cause of human behavior. Thus, behaviorism uses but does not strictly support psychological hedonism over other understandings of the ultimate drive of human behavior. Psychological egoism is controversial. Proponents cite evidence from introspection: reflection on one's own actions may reveal their motives and intended results to be based on self-interest. Psychological egoists and hedonists have found through numerous observations of natural human behavior that behavior can be manipulated through reward and punishment both of which have direct effects of pain and pleasure. Also, the work of some social scientists has empirically supported this theory. Further, they claim psychological egoism posits a theory that is a more parsimonious explanation than competing theories. Opponents have argued that psychological egoism is not more parsimonious than other theories. For example, a theory that claims altruism occurs for the sake of altruism explains altruism with less complexity than the egoistic approach. The psychological egoist asserts humans act altruistically for selfish reasons even when cost of the altruistic action is far outweighed by the reward of acting selfishly because altruism is performed to fulfill the desire of a person to act altruistically. Other critics argue that it is false either because it is an over-simplified interpretation of behavior or that there exists empirical evidence of altruistic behaviour. Recently, some have argued that evolutionary theory provides evidence against it.