The third volume in a projected four-part series on the evolution of human intelligence, a series acknowledged by opponents and supporters alike as one of the most important recent investigations into the sources and nature of human intelligence.

Professor Itzkoff first sets forth the evolutionary facts as a beacon to illuminate his biological theory of variable intelligence. Filling out the evidence from the psychological and brain sciences presented in THE FORM OF MAN and TRIUMPH OF THE INTELLIGENT, he here develops a full-scale model of the mind that more fully explains the relationship between I.Q. and the broader expressions of human intelligence as experienced in daily life.

(His model of variable intelligence further delves into the historical consequences of our ancient biological and psychoneurological heritage. Issues such as gender, race, and ethnic variability are treated factually and responsibly, taking into account the full range of empirical evidence available to scholars.)

The result is a theory having three complementary bodies of evidence - evolutionary, psychoneurological, and historical - building into a perspective on the factual «why» of human intellectual differences. These differences can best be observed in the richness and diversity of individual personality as well as in the cultures of our world. Itzkoff here takes the first tentative steps toward a biological explanation of the nature and structure of culture.

WHY HUMANS VARY IN INTELLIGENCE is an appeal for the rational study of human nature, a plea to allow for further nonideological research into this most crucial area of contemporary concern.