Gross Domestic Product (GDP), especially per capita GDP, is sometimes used to describe economic inequality and stratification at the international or global level. Social Social variables, both quantitative and qualitative, typically provide the most explanatory power in causal research regarding social stratification, either as independent variables or as intervening variables. Three important social variables include gender, race, and ethnicity, which, at the least, have an intervening effect on social status and stratification in most places throughout the world. Additional variables include those that describe other ascribed and achieved characteristics such as occupation and skill levels, age, education level, education level of parents, and geographic area. Some of these variables may have both causal and intervening effects on social status and stratification. For example, absolute age may cause a low income if one is too young or too old to perform productive work. The social perception of age and its role in the workplace, which may lead to ageism, typically has an intervening effect on employment and income. Social scientists are sometimes interested in quantifying the degree of economic stratification between different social categories, such as men and women, or workers with different levels of education. An index of stratification has been recently proposed by Zhou for this purpose. Gender Gender is one of the most pervasive and prevalent social characteristics which people use to make social distinctions between individuals. Gender distinctions are found in economic-, kinship- and caste-based stratification systems. Social role expectations often form along sex and gender lines. Entire societies may be classified by social scientists according to the rights and privileges afforded to men or women, especially those associated with ownership and inheritance of property. In patriarchal societies, such rights and privileges are normatively granted to men over women; in matriarchal societies, the opposite holds true. Sex- and gender-based division of labor is historically found in the annals of most societies and such divisions increased with the advent of industrialization. Sex-based wage discrimination exists in some societies such that men, typically, receive higher wages than women for the same type of work. Other differences in employment between men and women lead to an overall gender-based pay-gap in many societies, where women as a category earn less than men due to the types of jobs which women are offered and take, as well as to differences in the number of hours worked by women. These and other gender-related