the way in which the
            wealth and income of a nation are divided among its population, or the way in which the
            wealth and income of the world are divided among nations. Such patterns of distribution
            are discerned and studied by various statistical means, based on data of varying degrees
            of reliability.
            Wealth is an accumulated store
            of possessions and financial claims. It may be given a monetary value if prices can be
            determined for each of the possessions; this process can be difficult when the possessions
            are such that they are not likely to be offered for sale. Income is a net total of the
            flow of payments received in a given time period. Some countries collect statistics on
            wealth from legally required evaluations of the estates of deceased persons, which may or
            may not be indicative of what is possessed by the living. In many countries, annual tax
            statements that measure income provide more or less reliable information. Differences in
            definitions of income--whether, for example, income should include payments that are
            transfers rather than the result of productive activity, or capital gains or losses that
            change the value of an individual's wealth--make comparisons difficult.
            In order to classify patterns
            of national wealth and income, a basis of classification must be determined. Factor shares
            categorize wealth and income on the basis of the ownership of factors of production:
            labour, land, capital, and, occasionally, entrepreneurship, whose respective forms of
            income are labeled wages, rent, interest, and profit. Personal distribution statistics,
            usually developed from tax reports, categorize wealth and income on a per capita basis.
            Gross national product (GNP)
            per capita provides a rough measure of annual national income per person in different
            countries. Countries that have a sizable modern industrial sector have a much higher GNP
            per capita than countries that are undeveloped. In the late 1980s, for example, the World
            Bank estimated that developed countries had a GNP per capita of $17,000, while undeveloped
            countries had less than $400. Income also varies greatly within countries. In the United
            States there is considerable variation between agricultural and industrial regions,
            females and males, and nonwhites and whites. While the bulk of the U.S. population has a
            middle income that is derived largely from earnings, wages can vary considerably among
            occupations.
            A considerable proportion of
            high incomes derives from investment, and the higher the income, the higher the
            investment-derived portion tends to be. Because most fortunes require long periods to
            accumulate, the continuing existence of a class of very wealthy persons depends on their
            ability to keep their fortunes large and to pass them on to descendants. Earned incomes
            are affected by a different sort of inheritance. Access to well-paid jobs and to other
            occupations of high status is largely the product of education and opportunity. Typically,
            therefore, children tend to retain the status of their parents and are likely to earn
            similar incomes. Statistical studies of the distribution of wealth and income may shed
            light on various economic, social, and political questions.