In modern Western societies, social stratification typically is distinguished as three social classes: (i) the upper class, (ii) the middle class, and (iii) the lower class; in turn, each class can be subdivided into strata, e.g. the upper-stratum, the middle-stratum, and the lower stratum. Moreover, a social stratum can be formed upon the bases of kinship, clan, tribe or caste, or all four.
The categorization of people by social strata occurs in all societies, ranging from the complex, state-based or polycentric societies to tribal and feudal societies, which are based upon socio-economic relations among classes of nobility and classes of peasants. Historically, whether or not hunter-gatherer societies can be defined as socially stratified or if social stratification began with agriculture and common acts of social exchange, remains a debated matter in the social sciences. Determining the structures of social stratification arises from inequalities of status among persons, therefore, the degree of social inequality determines a person's social stratum. Generally, the greater the social complexity of a society, the more social strata exist, by way of social differentiation.
Social stratification is a term used in the social sciences to describe the relative social position of persons in a given social group, category, geographical region or other social unit. It derives from the Latin strātum (plural strata; parallel, horizontal layers) referring to a given society’s categorization of its people into rankings of socioeconomic tiers based on factors like wealth, income, social status, occupation and power. In modern Western societies, stratification is often broadly classified into three major divisions of social class: upper class, middle class, and lower class. Each of these classes can be further subdivided into smaller classes (e.g. "upper middle"). Social strata may also be delineated on the basis of kinship ties or caste relations.
The concept of social stratification is often used and interpreted differently within specific theories. In sociology, for example, proponents of action theory have suggested that social stratification is commonly found in developed societies, wherein a dominance hierarchy may be necessary in order to maintain social order and provide a stable social structure. So-called conflict theories, such as Marxism, point to the inaccessibility of resources and lack of social mobility found in stratified societies. Many sociological theorists have criticized the extent to which the working classes are unlikely to advance socioeconomically while the wealthy tend to hold political power which they use to exploit the proletariat (laboring class). Talcott Parsons, an American sociologist, asserted that stability and social order are regulated, in part, by universal values. Such values are not identical with "consensus" but can as well be an impetus for ardent social conflict as it has been multiple times through history. Parsons never claimed that universal values, in and by themselves, "satisfied" the functional prerequisites of a society. Indeed, the constitution of society is a much more complicated codification of emerging historical factors. Theorists such as Ralf Dahrendorf alternately note the tendency toward an enlarged middle-class in modern Western societies due to the necessity of an educated workforce in technological economies. Various social and political perspectives concerning globalization, such as dependency theory, suggest that these effects are due to change in the status of workers to the third world.
Four principles are posited to underlie social stratification. First, social stratification is socially defined as a property of a society rather than individuals in that society. Second, social stratification is reproduced from generation to generation. Third, social stratification is universal (found in every society) but variable (differs across time and place). Fourth, social stratification involves not just quantitative inequality but qualitative beliefs and attitudes about social status.
Although stratification is not limited to complex societies, all complex societies exhibit features of stratification. In any complex society, the total stock of valued goods is distributed unequally, wherein the most privileged individuals and families enjoy a disproportionate share of income, power, and other valued resources. The term "stratification system" is sometimes used to refer to the complex social relationships and social structure that generate these observed inequalities. The key components of such systems are: (a) social-institutional processes that define certain types of goods as valuable and desirable, (b) the rules of allocation that distribute goods and resources across various positions in the division of labor (e.g., physician, farmer, ‘housewife’), and (c) the social mobility processes that link individuals to positions and thereby generate unequal control over valued resources.
Social mobility is the movement of individuals, social groups or categories of people between the layers or strata in a stratification system. This movement can be intragenerational (within a generation) or intergenerational (between two or more generations). Such mobility is sometimes used to classify different systems of social stratification. Open stratification systems are those that allow for mobility between strata, typically by placing value on the achieved status characteristics of individuals. Those societies having the highest levels of intragenerational mobility are considered to be the most open and malleable systems of stratification. Those systems in which there is little to no mobility, even on an intergenerational basis, are considered closed stratification systems. For example, in caste systems, all aspects of social status are ascribed, such that one's social position at birth is the position one holds for a lifetime.