Margaret Sanger, Michael W. Perry, H. G. Wells (2003). “The Pivot of Civilization in Historical Perspective: The Birth Control Classic”, p.403, Inkling Books
"The development of intelligence in children" by Alfred Binet, Théodore Simon, translated by Elizabeth S. Kite, Reprinted 1973, New York: Arno Press, (pp. 42-43), 1983.