The Bell Curve: Intelligence and Class Structure in American Life, by Richard J. Herrnstein and Charles Murray, New York: The Free Press, 845 pages, $30.00

Charles Murray and the late Richard Herrnstein have produced a controversial and well-written book about human differences, the sources of human differences, and how we should respond to those differences. The early reactions to the book in the popular press have been emotional and denunciatory, focusing almost exclusively on the authors' discussion of racial differences and the genetic basis for those differences. This is unfortunate. The book is not devoted exclusively to a discussion of racial differences, although it certainly considers them in detail. It is obvious that most reviewers of the book have not read it as a whole, if they have read it at all. It is also clear that in an age of rampant egalitarianism, discussion of differences in cognitive skills remains taboo. The authors deserve much praise for discussing a forbidden subject and thereby initiating a public discussion that challenges the egalitarian presumptions of our day.

Like Robert Reich in The Work of Nations and Mickey Kaus in The End of Equality, the authors are concerned about the growth of economic and social inequality in American society, a topic that dominates many contemporary political discussions. Unlike those authors, Murray and Herrnstein probe more deeply into the personal sources of inequality, devoting considerable attention to the genetic component of personal differences and presenting fresh empirical evidence about an important relationship between their measure of IQ and success in society at large. Like Reich and Kaus, Murray and Herrnstein worry about the consequences for the social order of the growing inequality in economic and social success between the "haves" and the "have nots," and the social and economic partitioning of high-skill, high-IQ persons away from low-skill, low-IQ persons.

This 845-page book covers an enormous and impressive range of topics. Its numerous tables and charts make close reading a challenge. Indeed, all but trained social scientists will be intimidated by the statistical details and by the complicated arguments used by the authors. Even more forbidding to most readers will be the hundreds of pages of footnotes and appendix tables that document the statistical analysis underlying the arguments in the text. Despite all this, the book is organized in easily summarized sections. It is accessible at one level to all readers who are willing to skip the details.

The book contains four parts. Part I updates Herrnstein's 1973 book IQ and the Meritocracy and documents that American society has become more stratified on the basis of intelligence than it was even one generation ago. Merit—treated here as synonymous with IQ—has become concentrated in schools and the workplace. This increase in cognitive stratification results from the realization of the meritocratic vision of access to institutions based on individual ability. Social class and parental income play weaker roles in regulating access to education and jobs than at any time in American history.

The authors go on to note a phenomenon not discussed in Herrnstein's book—that since the late 1970s, the economic returns to measured skills, and in particular education, have increased. This has created a growing gap between the wages and employment of the skilled and the unskilled. The authors note a strong, but by no means perfect, relationship between skill and IQ.

Part II presents original empirical research, combined with a synthesis of the existing empirical literature, that shows a strong relationship between the authors' measure of IQ and social performance. This portion of the book puts empirical flesh on Herrnstein's original bare-bones argument. Low-IQ persons are more likely to be in poverty, drop out of school, be unemployed or altogether idle, be on welfare, be bad parents, commit crimes, and withdraw from political activity than are high-IQ persons. In general, this relationship holds even after adjusting for the authors' measure of socioeconomic background.

The authors wish their readers to draw from this exercise the conclusion that nature—not just parental or social environment—plays an important role in explaining a variety of social pathologies. Taken literally, their research demonstrates that IQ, rather than socioeconomic background, plays the dominant role in generating differences in a variety of socially important outcomes among persons. The analysis uses data only for whites. By proceeding in this way, the authors establish the importance of IQ in accounting for individual differences without getting into the controversial issue of racial bias in IQ tests.

In Part III, they mention the unmentionable by directly analyzing the sources of ethnic differences in social outcomes and the role of their measure of IQ in accounting for these differences. They firmly and rather convincingly refute the critics of IQ and aptitude tests who claim that the tests are racially biased and unrelated to true productivity in schools or the workplace. They discuss the well-documented disparity between the distributions of IQ for blacks and whites, along with other ethnic disparities. Their empirical work substantiates the role of IQ in accounting for a considerable portion of ethnic differences in socioeconomic outcomes and demonstrates the concentration of low-IQ persons (of all races) in a variety of pathological categories. The higher rate of reproduction and immigration among the lower-IQ groups also receives attention, along with the consequences of this phenomenon for the American gene pool. They claim that the average IQ is declining in the United States.

Part IV really consists of two separate sections. The first section builds on the first three parts of the book and discusses the implications of the authors' findings for social policy. Murray and Herrnstein present a pessimistic summary of efforts to raise cognitive ability through social programs. This review of the ineffectiveness of most social programs harkens back to Murray's Losing Ground, except that now the cognitive limitations of individuals rather than perverse incentives created by the programs lead to their failure. The authors discuss the "dumbing down" of American public education and the shift in educational expenditures away from gifted children. Under the aegis of promoting equality, Rawlsian educational policy has taken resources away from the able and given them to the less able.

They also discuss affirmative action in colleges and the workplace. Murray and Herrnstein make the simple, powerful, and apparently very controversial point that disparities in intelligence and abilities among ethnic groups, combined with equality of opportunity at the individual level, will produce demographic disparities in college attendance, job hiring, and promotion rates. Such disparities often lead to interventions by governments enforcing anti-discrimination laws.

Murray and Herrnstein argue convincingly that employment tests banned by the courts as discriminatory at least partly predict productivity and are, if anything, biased in favor of minorities. Further, they make the claim that prohibitions against using the tests impair economic productivity and that "race-normed" adjustments of test scores misclassify workers, create tokenism in the workplace, and often stigmatize the intended recipients of government beneficence. The press has attacked this section of the book, as well as the section on racial differences in IQ, as racist in tone and content. In fact, the authors advocate the nonracist policy of treating persons as individuals rather than as members of racial groups.

The last two chapters of the book abandon the empirical focus. The penultimate chapter presents a bleak vision of an IQ-stratified meritocracy with a cognitive elite increasingly isolated from the rest of society. In this worst-case scenario, the affluent and the cognitive elite merge interests, in part because many members of the cognitive elite have become affluent. Together they form a ruling class. A deteriorating quality of life emerges for the cognitively feeble, who become economically and socially dysfunctional. Unable to cope with the complexity of modern society, they become wards of the state.

In the final chapter, the authors turn to Murray's In Pursuit as an alternative to their bleak vision of a cognitively stratified social order. They harken to a communitarian ideal in which places are found for all persons in cognitively integrated local neighborhoods. Following Murray's previous book, they suggest that removing power from the center and returning it to the community will produce vital neighborhoods that will find a place for everyone and foster dignity and self-respect. Simple rules that everyone can understand, and simple morality and justice that clearly define what is right and wrong, are the essential features of a viable communitarian social order comprising persons with diverse cognitive skills.

A rigorous, well-reasoned challenge to contemporary presumptions about equality, egalitarianism, and the malleability of human beings is long overdue. Had the authors taken more care in presenting their evidence and summarizing that of others, and had they woven their argument more closely, their book would be that challenge. Unfortunately, it is not.

The book fails for four main reasons. First, too much space is devoted to discussions of intrinsically irrelevant issues. Nothing central to the case for recognizing diversity in human abilities hinges on the issue of whether there is one "true ability" or whether there are multiple abilities—as common sense, much psychometric research, and the authors' own evidence indicate is the actual state of affairs. Despite this evidence, Murray and Herrnstein devote many pages to justifying a one-ability, or "g," model of human intelligence. Admitting that persons have multiple skills does not undermine the empirical case that heterogeneity in ability is an important fact of social and economic life. Indeed, acknowledging a multiplicity of skills emphasizes human diversity.

The long discussions of heredity also distract attention away from the main thrust of the argument and generate needless controversy. The authors acknowledge, as does most serious science on the matter, the difficulty of identifying separate genetic and environmental contributions to intelligence. Most scholars assign some weight to both sources, but the allocation of precise weights generates much well-deserved controversy. The authors fail to justify why it is useful to establish any particular set of weights or even a range of weights, except the special weight that assigns all credit to the genes.

This observation points to the second, more fundamental, reason why this book fails to provide an effective challenge to contemporary egalitarian social policy. One might oppose such policies on moral or ethical grounds. Instead, the authors choose an empirical approach. Yet they fail to develop the empirical case in a satisfactory or coherent manner.

Before I read this book, I thought that Murray and Herrnstein would do for policies aimed at reducing inequality what Murray did for poverty policy in Losing Ground, by documenting the failure of many social programs designed to boost the skills of the less able. Chapter 17 of their book discusses the mixed evidence on the success of early childhood interventions designed to boost IQ. By no means does the evidence they discuss rule out the possibility of boosting IQ through programs that enrich the learning environments of young children. Indeed, the authors acknowledge that there are strong indications that very intensive programs can be effective. Half-hearted interventions like Head Start are definitely not effective.

It is striking that the authors do not discuss the costs and benefits of various interventions. It is in these terms that public policy discussions regarding skill-enhancement programs are usually conducted. The authors seek to short-circuit all of the hard work required to make credible cost-benefit calculations by claiming that there is a genetic basis for skill differences.

But estimates of a genetic component of skills are irrelevant to the requisite cost-benefit analysis unless it can be established that all differences are genetic. No one, including the authors, claims that this is so. Only if all differences are genetic—and if no offsets to genetic endowments can be created by families, communities, or governments—would it be possible to use the information on the genetic contribution of skills to make the required cost-benefit analysis. In that extreme case, which no one accepts as empirically relevant, the costs of making change are infinite. Genetics and heritability determine all. Social programs cannot be effective.

Equally obvious is the point that knowing that all skill components are environmentally determined does not justify interventions. Knowing that we can teach calculus to a child with an IQ of 65 but only at an enormous cost would not justify a policy of doing so, except in the minds of zealots committed to extreme egalitarian visions. Discussions of nature versus nurture are irrelevant to practical policy discussions couched in terms of costs and benefits. An evaluation of the effectiveness of interventions is required to justify or oppose social policy designed to cope with inequality. The authors do not provide a systematic evaluation of such programs. The Bell Curve fails to present the hard information required to settle these matters on the factual grounds chosen by the authors.

This point is particularly telling for their assessment of education. The authors offer an inconsistent treatment of education throughout the book. Early on, they highlight the finding of many recent studies—that the economic return to education has increased in the past 15 years or so. The gaps between the wages of college-educated workers and less-educated workers have widened. This phenomenon has contributed to growing income inequality.

They also establish that more-able persons tend to attain higher levels of schooling. They acknowledge—and then go on to forget—that the relationship between education and ability is far from exact. In fact, throughout much of the book, they equate ability and education and implicitly assume that the economic returns to ability drive the economic returns to education.

On the empirical grounds chosen by the authors, this implicit assumption is false. Their own evidence (buried in Appendix 6), as well as a vast literature in empirical social science, clearly indicates that controlling for ability lowers but does not eliminate the return to schooling measured in terms of earnings. The evidence on this point is consistent across many studies. Controlling for their measure of ability, the returns to education sometimes fall by as much as 25 percent, but they never go to zero.

Ability and education are not the same thing, and both have economic rewards. Accounting for ability weakens but hardly eliminates the role of education in raising earnings. On average, an extra year of schooling still increases earnings by at least a substantial 6 percent to 8 percent. So there is room for social policy to eliminate earnings differentials between persons of the same ability level. Neither The Bell Curve nor the literature on schooling provides much evidence on the all-important question of the efficacy of education as a tool for equalizing the earnings of persons of different ability levels.

What little is known indicates that ability—or IQ—is not a fixed trait for the young (persons up to age 8 or so). Herrnstein noted this in IQ and the Meritocracy. Sustained high-intensity investments in the education of young children, including such parental activities as reading and responding to children, stimulate learning and further education. Good environments promote learning for young children at all levels of ability. In this sense, there is fragmentary evidence that enriched education can be a good investment even for children of low initial ability, because it stimulates cumulative learning processes and may raise ability. There is much more negative evidence for adults, where ability is a more stable trait. For low-ability adults, there is little evidence that educational investments are socially profitable.

The authors also disregard much recent empirical evidence by Richard Murnane and others that indicates the increasing returns to the measures used by Murray and Herrnstein account for only a small portion of the recent increase in the economic return to schooling. While payments for ability have increased somewhat in the past 15 years, there remains a substantial increase in the payment for education unrelated to the authors' measure of ability.

It is unfortunate that the authors disregard this important evidence. Operating on the empirical playing field chosen by Murray and Herrnstein, a die-hard interventionist could find much credible evidence to support an active social policy to eliminate skill differentials. Their implicit claim that ability drives the economic return to education, and the recent increase in the economic return to education, fails to pass empirical muster.

The third source of The Bell Curve's failure lies in the details of its analysis of the impact of ability on measured outcomes such as earnings. In their empirical research, the authors examine how well one measure of ability explains a variety of economic and social behaviors. They pit their ability measure against a measure of the socioeconomic status of persons when they were children. The authors intend this contrast to reveal the relative importance of "genes" and "environment" in accounting for behavior. Outcomes are much more sensitive to their measure of ability than to their measure of socioeconomic status. Large changes in the socioeconomic variables have weak effects on the outcome measures, while small changes in ability have large effects on the same outcome measures.

This sort of empirical exercise prospers—or founders—on the details. The credibility of any empirical study depends on the care taken by the analyst in defining and measuring concepts, and in interpreting conclusions drawn from the data. It is at this point that the book becomes a policy polemic rather than a scholarly study of human differences.

First, consider the definitions of the two key variables used in the authors' empirical study: IQ and family background. In their empirical analysis, the operational definition of IQ is the Armed Forces Qualifying Test (AFQT) score, which is based on an aggregation of a subset of 10 separate exams given to more than 12,000 youth in 1980. The youth were 15 to 23 years old when the tests were administered. These tests were designed to predict success in military training schools, and there is much evidence that they do so, although by no means are they 100-percent reliable. Most of these tests appear to be achievement tests rather than ability tests (i.e., they partly measure factual knowledge and not pure ability). A subset of four tests was assembled to define the Armed Forces Qualifying Test (AFQT) score used by Murray and Herrnstein.

Ironically, the authors delete from their composite AFQT score a timed test of numerical operations because it is not highly correlated with the other tests. Yet it is well known that in the data they use, this subtest is the single best predictor of earnings of all the AFQT test components. The fact that many of the subtests are only weakly correlated with each other, and that the best predictor of earnings is only weakly correlated with their "g-loaded" score, only heightens doubts that a single-ability model is a satisfactory description of human intelligence. It also drives home the point that the "g-loading" so strongly emphasized by Murray and Herrnstein measures only agreement among tests—not predictive power for socioeconomic outcomes. By the same token, one could also argue that the authors have biased their empirical analysis against the conclusions they obtain by disregarding the test with the greatest predictive power.

More disturbing is the authors' treatment of family background. The index is based on parental education and occupational status, and on family income measured at one point in the entire life cycle of the child. For many young adults, the family-income measure is entirely missing and is omitted from the construction of the index.

The IQ measure used by Murray and Herrnstein is taken rather late in the life cycle of the child. (Recall that the IQ test is administered to youth 15 to 23 years old.) Many analysts suspect that IQ as measured by tests administered after early childhood reflects the outcome of social and cultural influences. The authors attempt to eliminate these influences by a standard statistical method called regression analysis.

But the standard statistical methods used by Murray and Herrnstein are vulnerable to measurement error. It would be incredible if 15 to 23 years of environmental influences, including the nurturing of parents, the resources they spent on a child, their cultural environment, their interaction with their children, and the influence of the larger community could be summarized by a single measure of education, occupation, and family income in one year. If environment is poorly measured but affects the test score—and there is solid evidence of environmental impacts on test scores—then Murray and Herrnstein's finding that IQ has a stronger impact on socioeconomic outcomes than the measured environment may simply arise from the poor quality of the measure of the environment.

The authors present evidence that IQ rises with age and with years of schooling completed. IQ may actually be a better measure of the environment facing children than the measure of environment used by Murray and Herrnstein. They use IQ to predict schooling, but schooling produces IQ. Hence, they are especially likely to find a strong measured effect of "IQ" on schooling.

The same remarks apply to their study of racial and ethnic differentials in socioeconomic outcomes. If racial differentials in environments affect ability and influence measured test scores, evidence that racial differentials weaken when ability is controlled for using regression methods does not rule out an important role for the environment in explaining performance in society. In the presence of measurement error in the environment, the authors' analysis will overstate the "true" effect of ability on those outcomes.

There are methods for addressing these problems, but Murray and Herrnstein do not use them. They should have tried a variety of measures of family background to explore the sensitivity of their reported results to the particular measures of family background they do use. A strict environmentalist could justifiably argue that the evidence reported by Murray and Herrnstein simply reveals the crudity of their measure of the environment and the strength of the correlation between the test score and their measure of environment.

One important technical point worth making here concerns the method used by the authors to measure standardized changes in IQ and family background. By its very construction as a measure that follows a bell curve, the "two-standard deviation" range in measured IQ used by the authors to gauge the sensitivity of outcomes to IQ represents a change in IQ ranging over 95 percent of the population. A "two-standard deviation" range of their family background index does not include 95 percent of the population, because that measure does not come from a bell curve. It may include as little as 75 percent of the population. By restricting the range of the environmental variable they understate the role of the environment in affecting outcomes relative to the role allocated to IQ.

Finally, the book fails due to a lack of coherence. The argument does not cumulate in a convincing way. Too many seams are visible. Its case against affirmative action and egalitarianism in education, and in favor of the use of testing in the workplace, does not require acceptance of a single ("g-loaded") scale of ability, or acceptance of the importance of heredity in producing socioeconomic inequality. The authors' evidence of growing stratification by cognitive ability in schools and workplaces does not require that they take a position on how the ability is produced. Yet the authors argue strenuously for their narrow view of ability and its heritability and thereby distract the reader's attention.

Nor do the two competing visions of the future of American society offered up in Part IV naturally flow from the arguments and evidence presented in the earlier parts of the book. The first dystopic vision relies on stronger sorting and heritability mechanisms than the authors have demonstrated actually operate in American society. Even if IQ is largely inherited, there is considerable scope for intergenerational economic mobility. The extreme pessimism of this scenario ignores the warnings issued by the authors that even among persons in the lowest ability grouping, there is still a lot of socially productive behavior. Their pessimistic vision relies on unsupported assumptions about the skill bias of future technological change and the inability of entrepreneurs—and social institutions—to efficiently utilize unskilled labor. This vision might be realized, but it reads more like a story borrowed from science fiction novels than a plausible extrapolation of existing social trends.

The second vision of communitarian harmony presented in the final chapter of the book is based on several implicit assumptions. The authors take it as self-evident that cognitively heterogeneous groups benefit both able and less-able members. That is an empirical statement about which the authors offer no evidence. If such heterogeneous associations are not mutually beneficial, strong forces of self-interest would arise that would promote segregation and separation along cognitive lines. In that case, their communitarian society of neighborhoods would only be viable if coercion were applied from central governments.

Cognitive stratification may be both economically efficient and socially desirable. Our cognitively stratified society may be a consequence of private choice, not governmental coercion. Murray and Herrnstein never discuss mechanisms or incentives that would lead persons to voluntarily embrace cognitively integrated local neighborhoods. They take it as self-evident that such neighborhoods would spontaneously emerge if central governments quit interfering in local communities.

Many of the other policy goals advocated by the authors could be embraced without any consideration of the problems—or benefits—of cognitive stratification. For this reason, their advocacy of these goals is not germane to this book. Simplifying the law, eliminating regulation, providing clearer moral and social rules, and eliminating restrictions on entry into business may differentially benefit the cognitively weak, but they are likely to benefit everyone else, too.

The authors' argument in support of local neighborhoods and small communities with cognitively mixed populations cries out for clarification. For each idyllic fable about the virtues of life in small communities, one could counter with fables of Peyton Place or the narrow-mindedness of the Babbitts of Main Street.

Had the authors been more cautious, they would have told the following defensible story: They have produced very convincing evidence that by the late teenage years, essential features of the skills and motivation of persons are determined. These features strongly influence individuals' performance in schools, in the market, and in other aspects of social life. The Armed Forces Qualifying Test seems to be a good measure of the skills affecting social performance. Using the components on which the test is based, rather than one composite score, would probably capture the diversity of abilities in the population even better.

The authors have no good way to separate genetic from social influences on social behavior. Their environmental data are too crude and the AFQT score they use is obtained too late in life to make a genetic-environmental distinction meaningful. The authors would require much finer measures of environmental variables than they have at their disposal to rule out the importance of family and society in determining individual outcomes.

Nonetheless, their evidence and the evidence assembled from many government skill-remediation programs for adults suggests that persons are not very malleable after their late teens or perhaps their early 20s. Successful interventions for such people are likely to be very costly. The literature suggests a particularly poor performance of educational remediation programs for adults of low cognitive ability as measured by AFQT and other cognitive tests.

To the extent that social interventions can upgrade skills, they are most likely to be effective when they are applied to the young. The fragments of evidence summarized in Chapter 17 of the book, and other evidence from high-intensity enriched-environment programs, point in this direction. This evidence is also consistent with the work of Thomas Sowell, who stresses the role of culture and values in shaping the expectations and motivations of young children. Job training and education are generally wasted on low-IQ adults. For this group, subsidies for employment may be justified, especially if work improves social behavior or is valued for its own sake. Economic efficiency is promoted by investing in the young. There is much evidence that learning is a cumulative, dynamic process. Learning begets learning. It is much easier to galvanize a young child than an illiterate young adult.

Future research should focus on growth and development in measured ability prior to age 15 (the age of the youngest person in the Murray-Herrnstein sample), because existing research indicates that values are formed and cognition is developed prior to that age. Genes may play some role, but culture and environment also contribute to ability and motivation. Much serious research in psychology indicates that motivation and attitude are as important—and possibly more important—for success than is raw IQ.

As for social policy, we should recognize that heterogeneity in experiences and endowments produces a wide range of cognitive skills and motivations. For a variety of reasons, treating persons fairly as individuals may lead to heterogeneity in outcomes among demographic groups. Denying individual heterogeneity by treating persons as members of demographic categories will produce disparities in productivity among demographic groups, reduce economic efficiency, and foster a sense of injustice among all participants in society.

James J. Heckman is Henry Schultz Professor in the department of economics, director of the Center for Social Program Evaluation at the Irving B. Harris Graduate School of Public Policy Studies at the University of Chicago, and Fellow, American Bar Foundation.