It is important to distinguish between the distribution of social rewards (e.g., the income distribution) and the distribution of opportunities for securing these rewards. For many Americans, substantial inequalities of income or wealth are entirely unproblematic, but only insofar as they are the outcome of a fair contest in which everyone has an equal opportunity to come out ahead. The study of social mobility entails examining whether the contest is indeed fairly run. Are the facts of the mobility regime consistent, in other words, with our long-standing commitment to equal opportunity? This line of questioning is addressed by examining (a) overall rates of mobility, and (b) the causes and sources of social mobility.
How frequently do individuals move into new social classes, different occupations, or higher or lower income groups? Is there a permanent "underclass?" A permanent self-reproducing elite? Has the rise in income inequality been coupled with increasing mobility between income groups? Does the United States have a distinctively fluid mobility regime?
Why do some people move up and others move down? Does it help a lot to have rich parents or to know the right people? Are ongoing changes in the occupational structure (e.g., decline of blue-collar sector, growth of service sector) generating much upward and downward social mobility?