1. Armstrong, E. L., & Woodley, M. A. (under review). The rule-dependence model explains the commonalities between the Flynn effect and IQ gains via retesting.
2. Blythe, D. (1964). The age of illusion. Boston: Houghton Mifflin.
3. Brinton, C. (1959). A history of Western morals. New York: Harcourt.
4. Carey, J. (1992). The intellectuals and the masse: Pride and prejudice among the literary intelligentsia, 1880–1939. London: Faber.
5. Carrie, A. (2012). Occupation change: 1920–2010. Weldon Cooper Center for Public Service:
http://statchatva.org/2012/04/06/occupation-change-1920-2010/
Note: The 1900 census did not use the same system of classification. However, Carrie put 1920 at 5 percent professionals and 1910 was 4 percent: Durand, E. D., & Harris, W. J. (1999), Population 1910: Occupational statistics (United States Bureau of the Census), New York: Norman Ross (Table 14). Therefore, I put 1900 at 3 percent.