Social stratification involves categorizing society into socioeconomic classes based on factors like occupation, wealth, and power. There are generally considered to be three main classes: the upper class, middle class, and lower class. Political stratification is based on levels of power over others. Social mobility refers to movement between classes, either horizontally within a class or vertically to a different class. Social inequality results from unequal opportunities and rewards across social positions. Discrimination and prejudice against racial or ethnic groups often leads to racial/ethnic inequality. Major causes of social change include technology, social institutions, population changes, environmental shifts, and modernization.