Arthur Jensen has systematically developed a seminal concept first discovered by Charles Spearman in the 1920s: individual and group differences in mental ability exist, and these differences can be measured by a single, general factor, g. On its surface, this concept seems innocuous. However, Jensen does not draw back from its most controversial conclusions - that the average differences in IQ and other abilities found between sexes and racial groups have a substantial hereditary component, and that these differences have important societal consequences. The culmination of his career is the most comprehensive treatment of g ever written, The g Factor. In it, Dr. Jensen not only clearly explains the psychometric, statistical, genetic, and physiological basis of g, in the process he also refutes all major challenges that have been brought against the concept of general mental ability.