he Bell Curve is a very sober, very thorough, and very honest book (1.) - on a subject where sobriety, thoroughness, and honesty are only likely to provoke cries of outrage. Its authors, Charles Murray and the late Professor Richard J. Herrnstein of Harvard, must have known that writing about differences in intelligence would provoke shrill denunciations from some quarters. But they may not have expected quite so many, quite so loudly or venomously, and from such a wide spectrum of people who should know better.
The great danger in this emotional atmosphere is that there will develop a two-tiered set of reactions--violent public outcries against the message of The Bell Curve by some, and uncritical private acceptance of it by many others, who hear no rational arguments being used against it. Both reactions are unwarranted, but not unprecedented, in the over-heated environment surrounding so many touchy social issues today.
The predictive validity and social implications of intelligence test results are carefully explored by Herrnstein and Murray in more than 500 pages of text with another 300 pages of appendices, footnotes, and an index. The Bell Curve is an education on the whole subject, including the evidence pro and con on a wide variety of controversial issues. Even where the authors clearly come down on one side of a given issue, they usually present the case for believing otherwise. In such candor, as well as in the clarity with which technical issues are discussed without needless jargon, this book is a model that others might well emulate.
Contrary to much hysteria in the media, this is not a book about race, nor is it trying to prove that blacks are capable only of being hewers of wood and drawers of water. The first 12 chapters of the book deal solely with data from all-white samples, so as to be rid of the distracting issue of racial differences in IQ scores. In these chapters, Herrnstein and Murray establish their basic case that intelligence test scores are highly correlated with important social phenomena from academic success to infant mortality, which is far higher among babies whose mothers are in the bottom quarter of the IQ distribution.
Empirical data from a wide variety of sources establish that even the differing educational backgrounds or socioeconomic levels of families in which individuals were raised are not as good predictors of future income, academic success, job performance ratings, or even divorce rates, as IQ scores are. It is not that IQ results are infallible, or even that correlations between IQ and these other social phenomena are high. Rather, the correlations simply tend to be higher than correlations involving other factors that might seem more relevant on the surface. Even in non-intellectual occupations, pen-and-paper tests of general mental ability produce higher correlations with future job performance than do "practical" tests of the particular skills involved in those jobs.
In such a comprehensive study of IQ scores and their social implications, there is no way to leave out questions of intergroup differences in IQ without the absence of such a discussion being glaring evidence of moral cowardice. After ignoring this issue for the first 12 chapters, Herrnstein and Murray enter into a discussion of it in Chapter 13 ("Ethnic Differences in Cognitive Ability"), not as zealots making a case but as researchers laying out the issues and reaching the conclusions that seem to them most consistent with the facts--while also presenting alternative explanations. They seem to conclude, however tentatively, that the apparent influence of biological inheritance on IQ score differences among members of the general society may also explain IQ differences between different racial and ethnic groups.
This is what set off the name-calling and mud-slinging with which so many critics of The Bell Curve have responded. Such responses, especially among black intellectuals and "leaders," are only likely to provoke others to conclude that they protesteth too much, lending more credence to the conclusion that genetics determines intelligence. Such a conclusion goes beyond what Herrnstein and Murray say, and much beyond what the facts will support.
First of all, Herrnstein and Murray make a clear distinction between saying that IQ is genetically inheritable among individuals in general and saying that the differences among particular groups are due to different genetic inheritances. They say further that the whole issue is "still riddled with more questions than answers." They caution against "taking the current ethnic differences as etched in stone." But none of this saves them from the wrath of those who promote the more "politically correct" view that the tests are culturally biased and lack predictive validity for non-white minorities.
It is an anomaly that there exists a controversy over the predictive validity of tests. This is ultimately an empirical question, one for which there is a vast amount of data going back many years. Herrnstein and Murray are only summarizing these data when they shoot down the arguments and evasions by which the conventional wisdom says that these tests do not accurately predict future performance. Long before The Bell Curve was published, the empirical literature showed repeatedly that IQ and other mental tests do not predict a lower subsequent performance for minorities than the performance that in fact emerges. In terms of logic and evidence, the predictive validity of mental tests is the issue least open to debate. On this question, Murray and Herrnstein are most clearly and completely correct.
In thus demolishing the foundation underlying such practices as double-standards in college admissions and "race-norming" of employment tests, The Bell Curve threatens both a whole generation of social policies and the careers of those who promote them. To those committed to such policies, this may be at least as bad as the authors remaining "agnostic" (as Herrnstein and Murray put it) on the question as to whether black- white IQ differences are genetic in origin.
On some other issues, however, the arguments and conclusions of The Bell Curve are much more open to dispute. Yet critics have largely overlooked these disputable points, while concentrating their attacks on either the unassailable conclusions of the book or the presumed bad intentions of the authors.
While Herrnstein and Murray do an excellent job of exposing the flaws in the argument that tests are culturally biased by showing that the greatest black-white differences are not on the questions which presuppose middle-class vocabulary or experiences, but on abstract questions such as spatial perceptual ability, their conclusion that this "phenomenon seems peculiarly concentrated in comparisons of ethnic groups" is simply wrong.
When European immigrant groups in the United States scored below the national average on mental tests, they scored lowest on the abstract parts of those tests. So did white mountaineer children in the United States tested back in the early 1930s. So did canal boat children in Britain, and so did rural British children compared to their urban counterparts, at a time before Britain had any significant non-white population. So did Gaelic-speaking children as compared to English-speaking children in the Hebrides Islands. This is neither a racial nor an ethnic peculiarity. It is a characteristic found among low-scoring groups of European as well as African ancestry.
In short, groups outside the cultural mainstream of contemporary Western society tend to do their worst on abstract questions, whatever their race might be. But to call this cultural "bias" is misleading, particularly if it suggests that these groups' "real" ability will produce better results than their test scores would indicate. That non sequitur was destroyed empirically long before Herrnstein and Murray sat down to write The Bell Curve. Whatever innate potential various groups may have, what they actually do will be done within some particular culture. That intractable reality cannot be circumvented by devising "culture-free" tests, for such tests would also be purpose-free in a world where there is no culture-free society.
Perhaps the strongest evidence against a genetic basis for intergroup differences in IQ is that the average level of mental test performance has changed very significantly for whole populations over time and, moreover, particular ethnic groups within the population have changed their relative positions during a period when there was very little intermarriage to change the genetic makeup of these groups.
While The Bell Curve cites the work of James R. Flynn, who found substantial increases in mental test performances from one generation to the next in a number of countries around the world, the authors seem not to acknowledge the devastating implications of that finding for the genetic theory of intergroup differences, or for their own reiteration of long-standing claims that the higher fertility of low-IQ groups implies a declining national IQ level. This latter claim is indeed logically consistent with the assumption that genetics is a major factor in interracial differences in IQ scores. But ultimately this too is an empirical issue--and empirical evidence has likewise refuted the claim that IQ test performance would decline over time.
Even before Professor Flynn's studies, mental test results from American soldiers tested in World War II showed that their performances on these tests were higher than the performances of American soldiers in World War I by the equivalent of about 12 IQ points. Perhaps the most dramatic changes were those in the mental test performances of Jews in the United States. The results of World War I mental tests conducted among American soldiers born in Russia--the great majority of whom were Jews--showed such low scores as to cause Carl Brigham, creator of the Scholastic Aptitude Test, to declare that these results "disprove the popular belief that the Jew is highly intelligent." Within a decade, however, Jews in the United States were scoring above the national average on mental tests, and the data in The Bell Curve indicate that they are now far above the national average in IQ.
Strangely, Herrnstein and Murray refer to "folklore" that "Jews and other immigrant groups were thought to be below average in intelligence. " It was neither folklore nor anything as subjective as thoughts. It was based on hard data, as hard as any data in The Bell Curve. These groups repeatedly tested below average on the mental tests of the World War I era, both in the army and in civilian life. For Jews, it is clear that later tests showed radically different results--during an era when there was very little intermarriage to change the genetic makeup of American Jews.
My own research of twenty years ago showed that the IQs of both Italian-Americans and Polish-Americans also rose substantially over a period of decades. Unfortunately, there are many statistical problems with these particular data, growing out of the conditions under which they were collected. However, while my data could never be used to compare the IQs of Polish and Italian children, whose IQ scores came from different schools, nevertheless the close similarity of their general patterns of IQ scores rising over time seems indicative--especially since it follows the rising patterns found among Jews and among American soldiers in general between the two world wars, as well as rising IQ scores in other countries around the world.
The implications of such rising patterns of mental test performance is devastating to the central hypothesis of those who have long expressed the same fear as Herrnstein and Murray, that the greater fertility of low-IQ groups would lower the national (and international) IQ over time. The logic of their argument seems so clear and compelling that the opposite empirical result should be considered a refutation of the assumptions behind that logic.
One of the reasons why widespread improvements in results on IQ tests have received such little attention is that these tests have been normed to produce an average IQ of 100, regardless of how many questions are answered correctly. Like "race-norming" today, such generation-norming, as it were, produces a wholly fictitious equality concealing very real and very consequential differences. If a man who scores 100 on an IQ test today is answering more questions correctly than his grandfather with the same IQ answered two-generations ago, then someone else who answers the same number of questions correctly today as this man's grandfather answered two generations ago may have an IQ of 85.
Herrnstein and Murray openly acknowledge such rises in IQ and christen them "the Flynn effect," in honor of Professor Flynn who discovered it. But they seem not to see how crucially it undermines the case for a genetic explanation of interracial IQ differences. They say:
The national averages have in fact changed by amounts that are comparable to the fifteen or so IQ points separating blacks and whites in America. To put it another way, on the average, whites today differ from whites, say, two generations ago as much as whites today differ from blacks today. Given their size and speed, the shifts in time necessarily have been due more to changes in the environment than to changes in the genes.
While this open presentation of evidence against the genetic basis of interracial IQ differences is admirable, the failure to draw the logical inference seems puzzling. Blacks today are just as racially different from whites of two generations ago as they are from whites today. Yet the data suggest that the number of questions that blacks answer correctly on IQ tests today is very similar to the number answered correctly by past generations of whites. If race A differs from race B in IQ, and two generations of race A differ from each other by the same amount, where is the logic in suggesting that the IQ differences are even partly racial?
Herrnstein and Murray do not address this question, but instead shift to a discussion of public policy:
Couldn't the mean of blacks move 15 points as well through environmental changes? There seems no reason why not--but also no reason to believe that white and Asian means can be made to stand still while the Flynn effect works its magic.
But the issue is not solely one of either predicting or controlling the future. It is a question of the validity of the conclusion that differences between genetically different groups are due to those genetic differences, whether in whole or in part. When any factor differs as much from Al to A2 as it does from A2 to B2, why should one conclude that this factor is due to the difference between A in general and B in general? That possibility is not precluded by the evidence, but neither does the evidence point in that direction.(2.)
A remarkable phenomenon commented on in the Moynihan report of thirty years ago goes unnoticed in The Bell Curve--the prevalence of females among blacks who score high on mental tests. Others who have done studies of high- IQ blacks have found several times as many females as males above the 120 IQ level. Since black males and black females have the same genetic inheritance, this substantial disparity must have some other roots, especially since it is not found in studies of high-IQ individuals in the general society, such as the famous Terman studies, which followed high-IQ children into adulthood and later life. If IQ differences of this magnitude can occur with no genetic difference at all, then it is more than mere speculation to say that some unusual environmental effects must be at work among blacks. However, these environmental effects need not be limited to blacks, for other low-IQ groups of European or other ancestries have likewise tended to have females over-represented among their higher scorers, even though the Terman studies of the general population found no such patterns.
One possibility is that females are more resistant to bad environmental conditions, as some other studies suggest. In any event, large sexual disparities in high-IQ individuals where there are no genetic or socioeconomic differences present a challenge to both the Herrnstein- Murray thesis and most of their critics.
Black males and black females are not the only groups to have significant IQ differences without any genetic differences. Identical twins with significantly different birthweights also have IQ differences, with the heavier twin averaging nearly 9 points higher IQ than the lighter one.(3.) This effect is not found where the lighter twin weighs at least six and a half pounds, suggesting that deprivation of nutrition must reach some threshold level before it has a permanent effect on the brain during its crucial early development.
Perhaps the most intellectually troubling aspect of The Bell Curve is the authors' uncritical approach to statistical correlations. One of the first things taught in introductory statistics is that correlation is not causation. It is also one of the first things forgotten, and one of the most widely ignored facts in public policy research. The statistical term "multicollinearity," dealing with spurious correlations, appears only once in this massive book.
Multicollinearity refers to the fact that many variables are highly correlated with one another, so that it is very easy to believe that a certain result comes from variable A, when in fact it is due to variable Z, with which A happens to be correlated. In real life, innumerable factors go together. An example I liked to use in class when teaching economics involved a study showing that economists with only a bachelor's degree had higher incomes than economists with a master's degree and that these in turn had higher incomes than economists with Ph.D.'s. The implication that more education in economics leads to lower incomes would lead me to speculate as to how much money it was costing a student just to be enrolled in my course. In this case, when other variables were taken into account, these spurious correlations disappeared.(4.) In many other cases, however, variables such as cultural influences cannot even be quantified, much less have their effects tested statistically.
The Bell Curve is really three books in one. It is a study of the general effects of IQ levels on the behavior and performance of people in general in a wide range of endeavors. Here it is on its most solid ground. It is also an attempt to understand the causes and social implications of IQ differences among ethnic groups. Here it is much more successful in analyzing the social implications where, as the authors say, "it matters little whether the genes are involved at all." Finally, it is a statement of grave concerns for the future of American society and a set of proposals as to how public policy should proceed in matters of education and social welfare programs. These concerns need voicing, even if they are not always compelling. One chance in five of disaster is not to be ignored. That is, after all, greater than the chance of disaster in playing Russian roulette.
In one sense, the issues are too important to ignore. In another sense the differences between what Herrnstein and Murray said and what others believe is much smaller than the latter seem to think. The notion that "genes are destiny" is one found among some of the more shrill critics, but not in The Bell Curve itself. Nor is race a kind of intellectual glass ceiling for individuals. As the authors write:
It should be no surprise to see (as one does every day) blacks functioning at high levels in every intellectually challenging field. Critics who insist on arguing that we are talking about an intellectual glass ceiling should recognize that this is their own straw man, not something from The Bell Curve. And if they refuse to recognize this, then we should recognize these critics as demagogues in the business of scavenging for grievances.
The Bell Curve deserves critical attention, not public smearing or uncritical private acceptance.
(1.) Editor's note: Our own review of The Bell Curve, by Christopher Caldwell, appeared last month. (2.) It is widely acknowledged that height is heavily influenced by genes, and it is not controversial that races differ in height because of these genetic differences. Yet predictions of a decline in national height over time, because of a greater fertility in groups of shorter stature, were likewise confounded by an increase in the national height. Yet, rightly, no one regards this as a refutation of the belief that height is greatly influenced by genes and differs from race to race for genetic reasons. Similarly, rising IQs over time do not refute the belief that races differ in IQ for genetic reasons, though it ought to at least raise a question about that belief. The parallel breaks down when we realize that height can be measured directly, as innate potential cannot be, but is wholly dependent on inferences about what would have happened in the absence of environmental differences. (3.) Miles D. Storfer, Intelligence and Giftedness: The Contributions of Heredity and Early Environment (San Francisco: Jossey-Bass Publishers, 1990), p. 13. (4.) Because a postgraduate degree was usually required to be an economist, those economists with only a bachelor's degree tended to have entered the profession before this requirement became common. That is, they tended to be older and have more experience, with this experience being more likely to have been in the more lucrative business world rather than in academia.