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2. 3. G. Masnik, and M.J. Bane (1980 ) The Nation's Families: 1960-1990 , p.4 . Joint Center for Urban Studies of MIT and Harvard University, Cambridge, MA.
3. 4. A. Smith (1937 ) The Wealth of Nations , p. 620 (Original work published 1776.) Modern Library Edition, Random House, New York.