Jensen provides a comprehensive treatment of one of the major constructs of behavioral science—general mental ability—labeled the g factor by its discoverer, Charles Spearman. The g factor is about individual differences in mental abilities. In factor analyses of any and every large and diverse collection of measures of mental abilities, however varied the content of knowledge and skills they call upon, g emerges as the largest, most general source of differences between individuals and between certain subpopulations.

Jensen fully and clearly explains the psychometric, statistical, genetic, and physiological basis of g, as well as the major theoretical challenges to the concept. For decades a key construct in differential psychology, the g factor's significance for scholars and researchers in the brain sciences as well as education, sociology, anthropology, evolutionary psychology, economics, and public policy is clearly evident in this, the most comprehensive treatment of g ever published.