Karl Krall, Denkende Tiere: Beiträge zur Tierseelenkunde auf Grund eigener Versuche (Leipzig: F. Engelmann, 1912).

Christopher B. Ruf, Erik Trinkaus and Trenton W. Holliday, ‘Body Mass and Encephalization in Pleistocene Homo’, Nature 387:6629 (1997), 173–176; Maciej Henneberg and Maryna Steyn, ‘Trends in Cranial Capacity and Cranial Index in Subsaharan Africa During the Holocene’, American Journal of Human Biology 5:4 (1993), 473–479; Drew H. Bailey and David C. Geary, ‘Hominid Brain Evolution: Testing Climatic, Ecological, and Social Competition Models’, Human Nature 20:1 (2009), 67–79; Daniel J. Wescott and Richard L. Jantz, ‘Assessing Craniofacial Secular Change in American Blacks and Whites Using Geometric Morphometry’, in Modern Morphometrics in Physical Anthropology: Developments in Primatology: Progress and Prospects, ed. Dennis E. Slice (New York: Plenum Publishers, 2005), 231–245.