ordinary language. IQ scores are stable, although not perfectly so, over much of a person's life. Properly administered IQ tests are not demonstrably biased against social, economic, ethnic, or racial groups. Cognitive ability is substantially heritable, apparently no less than 40 percent and no more than 80 percent. At the close of the introduction, the authors warn the reader against committing the ecological fallacy of inferring things about individuals based on the aggregate data presented in the book. They also assert that intelligence is just one of many valuable human attributes and one whose importance among human virtues is overrated. Part I. The Emergence of a Cognitive Elite In the first part of the book Herrnstein and Murray chart how American society was transformed in the 20th century. They argue that America evolved from a society where social origin largely determined one's social status to one where cognitive ability is the leading determinant of status. The growth in college attendance, a more efficient recruitment of cognitive ability, and the sorting of cognitive ability by selective colleges are identified as important drivers of this evolution. Herrnstein and Murray propose that the cognitive elite has been produced by a more technological society which offers enough high skill jobs for those with a higher intelligence to fill. They also propose that by removing race, gender or class as criteria (via the establishment of free primary education and the prohibition of discrimination) the main criteria of success in academic and professional life is becoming primarily based on cognitive ability. Increased occupational sorting by cognitive ability is discussed. They maintain that cognitive ability is the best predictor of worker productivity. Herrnstein and Murray argue that due to increasing returns to cognitive ability, a cognitive elite is being formed in America. They argue that this elite is getting richer and progressively more segregated from the rest of society. Part II. Cognitive Classes and Social Behavior The second part describes how cognitive ability is related to social behaviors: high ability predicts socially desirable behavior, low ability undesirable behavior. The argument is made that group differences in social outcomes are better explained by intelligence differences rather than socioeconomic status, a perspective, the authors argue, that has been neglected in research. The analyses reported in this part of the book were done using data from the National Longitudinal Survey of Labor Market Experience of Youth (NLSY), a study conducted