Egalitarianism is the moral doctrine that equality ought to prevail among some group along some dimension. One can best understand various types of egalitarianism by asking "Who is supposed to be equal?" and "In what respect are they supposed to be equal?"
Almost all theories of egalitarianism regard persons as the relevant group among whom equality should prevail. However, some versions of utilitarianism, such as Peter Singer's, include animals and maintain that the pleasures and pains of every animal, not only human animals, should count equally in moral deliberation. Singer has defended this view on what he calls the principle of equal consideration of interests.
Common forms of egalitarianism are material or economic egalitarianism, moral egalitarianism, legal egalitarianism, democratic egalitarianism, political egalitarianism, and opportunity egalitarianism. According to material egalitarianism, everyone ought to equal with respect to material possessions. According to legal egalitarianism, everyone ought to be considered equal under the law. According to moral egalitarianism, each person is of equal moral worth. According to democratic egalitarianism, everyone ought to have an equal voice in public affairs. According to political egalitarianism, everyone ought to be equal in political power. According to opportunity egalitarianism everyone ought to be equal in economic opportunity.
Kinds of egalitarianism can conflict. For instance, communism is an egalitarian doctrine according to which everyone in supposed to enjoy material equality. However, because material inequality is pervasive, any regime that wishes to erase these inequalities must set up a mechanism of material redistribution, and because people do not ordinarily part willingly with their property for redistributive purposes, this mechanism must be coercive. However, when coercive powers of redistribution are vested in some people and not in others, inequalities of political power emerge. History has shown, in the former Soviet Union for instance, that people who are granted coercive redistributive powers often abuse it. Indeed, those with political power were known to redistribute vastly unequal shares of material resources to themselves, thereby completely confounding the justification for their unequal political status. Therefore, some defenders of material egalitarianism have rejected Marxist communism in favor of such views as Libertarian socialism, which does not advocate the transitional use of the state as a means of redistribution.
The United States Declaration of Independence included a kind of moral and legal egalitarianism. Because "all men are created equal" the state is under an obligation to treat each person equally under the law. Originally this statement excluded women, slaves and other minority groups, but over time this kind of egalitarianism has won wide adherence and is a core component of liberal, democratic polities. Other kinds of egalitarianism are more controversial. Economic egalitarianism, popular on the Left throughought much of the 20th Century, has given way to a concern not that everyone be stictly equal in material possession, but rather that everyone be equal in having enough material goods to successfully fulfill his or her native human capacities. As long as everyone's basic needs are met, material inequality can flourish.
Libertarianism can be understood as radical political egalitarianism, according to which everyone is equal (or nearly equal) in coercive political power, because no one has any (or those who have it have little and are strictly limited in their use of it). However, political egalitarians, such as the libertarians, often face strong criticism from economic egalitarians who worry about the extremes of economic inequality made possible by unfettered markets.