Heer, David M.
Prentice-Hall, 1975
Buku Teks
edited by Philip M. Hauser and Otis Dudley Duncan
The University of Chicago Press, 1959
Buku Teks
Goldscheider, Calvin
Litle Brown, 1971
Buku Teks
edited by D. V. Glass and Roger Revelle
Edward Arnold, 1972
Buku Teks
Woytinsky, W.S.
The Twentieth Century Fund, 1953
Buku Teks