The Bell Curve


 * See normal distribution for the "bell curve" in statistics.

The Bell Curve is a controversial, best-selling 1994 book by the late Harvard professor Richard J. Herrnstein and American Enterprise Institute political scientist Charles Murray. Its central point is that intelligence is a better predictor of many factors including financial income, job performance, unwed pregnancy, and crime than parent's Socio-Economic status or education level. Also, the book argued that those with high intelligence (the "cognitive elite") are becoming separated from the general population of those with average and below-average intelligence, and that this was a dangerous social trend. Much of the controversy erupted from Chapters 13 and 14, where the authors write about the enduring differences in race and intelligence and discuss implications of that difference. The authors were reported throughout the popular press as arguing that these IQ differences are genetic, although they state no position on the issue in the book, and write in the introduction to Chapter 13 that "The debate about whether and how much genes and environment have to do with ethnic differences remains unresolved."

The conclusions made in the book are deducted from the research of others. Although Richard Herrnstein had a strong background in psychology, co-author Charles Murray was not an expert in intelligence testing, according to psychologist Leon J. Kamin

The book's title comes from the bell-shaped normal distribution of IQ scores. The Normal distribution is the limiting distribution of a random quantity which is the sum of smaller, inpendent random phenomena. The message in the title is that IQ scores are  Normally distributed  because a person's intelligence is the sum of many small random variations in genetic and environmental factors.

Shortly after publication, large numbers of people rallied both to criticize and defend the book. Some critics denounced the book and its authors as supporting scientific racism. A number of books were written in response, to criticize The Bell Curve. Those books included The Bell Curve Debate, which contains essays that respond to the controversial issues raised in The Bell Curve.

Content
The Bell Curve contains 845 pages in the first printing and 879 in the revised paperback. Much of its material is technical and academic, but the book's statistical explanations are styled to appeal to a general audience. There are extensive notes, graphs, and tables.

The Bell Curve is divided into four sections.
 * Part I argues that social stratification on the basis of intelligence has been increasing since the beginning of the twentieth century.
 * Part II presents original research showing significant correlations between intelligence and various social and economic outcomes. For instance, based on data as of 1989 this section shows that among non-Hispanic whites, intelligence level (cognitive class) is a better predictor of poverty than parents' socioeconomic class as shown on the summary table below:


 * {|class="wikitable" style="background:lavender;color:black"

! Category || Parents' socioeconomic class || Cognitive class
 * + Percentage in Poverty
 * Very high/Very bright || 3 || 2
 * High/Bright || 3 || 3
 * Average || 7 || 6
 * Low/Dull || 12 || 16
 * Very low/Very dull || 24 || 30
 * Overall average|| 7 || 7
 * }
 * Low/Dull || 12 || 16
 * Very low/Very dull || 24 || 30
 * Overall average|| 7 || 7
 * }
 * Overall average|| 7 || 7
 * }


 * Part III, by far the most controversial, examines what role IQ plays in contributing to social and economic differences between ethnic groups in America.
 * Part IV discusses the implications of the findings for education and social policy in the United States.

Herrnstein and Murray in many ways follow in the footsteps of UC Berkeley researcher Arthur Jensen, whose controversial article on the subject appeared in 1969 in the Harvard Educational Review. The Bell Curve argues that:
 * 1) Intelligence exists and is accurately measurable across racial, language, and national boundaries.
 * 2) Intelligence is one, if not the most, important correlative factor in economic, social, and overall success in America, and is becoming more important.
 * 3) Intelligence is largely (40% to 80%) genetically heritable.
 * 4) No one has so far been able to manipulate IQ long term to any significant degree through changes in environmental factors - except for child adoption - and in light of their failure such approaches are becoming less promising.
 * 5) The USA has been in denial regarding these facts, and in light of these findings a better public understanding of the nature of intelligence and its social correlates is necessary to guide future policy decisions in America.

Their evidence comes from an analysis of data compiled in the National Longitudinal Study of Youth (NLSY), a study conducted by the U.S. Department of Labor's Bureau of Labor Statistics tracking thousands of Americans starting in the 1980s. All participants in the NLSY took the Armed Forces Qualifying Test (AFQT), a measure of cognitive ability comparable to an IQ test. Participants were later evaluated for social and economic outcomes. In general, IQ/AFQT scores were a better predictor of life outcomes than social class background. Similarly, after statistically controlling for differences in IQ, many outcome differences between racial-ethnic groups disappeared. (See also Significance of group IQ differences.)

Policy recommendations
Herrnstein and Murray recommended the elimination of welfare programs (p. 548):


 * We can imagine no recommendation for using the government to manipulate fertility that does not have dangers. But this highlights the problem: The United States already has policies that inadvertently social-engineer who has babies, and it is encouraging the wrong women. If the United States did as much to encourage high-IQ women to have babies as it now does to encourage low-IQ women, it would rightly be described as engaging in aggressive manipulation of fertility. The technically precise description of America's fertility policy is that it subsidizes births among poor women, who are also disproportionately at the low end of the intelligence distribution. We urge generally that these policies, represented by the extensive network of cash and services for low-income women who have babies, be ended.

This claim spurred later research in economics and sexology, which considered that welfare programs for females had a doubly negative effect on aggregate IQ within the transfer group, by allowing the female partner to forgo a full consideration of the male's ability to serve as a provider of familial resources, instead placing greater emphasis on desirable physical or social characteristics (presumed to be not as positively correlated with IQ). Neither of these claims, as originally embodied in text and the follow-on research, dealt with race as such, but rather demonstrated concern that large numbers of minorities were positioned as recipients, leading to a continual worsening of the measured divergence in intelligence. However, two years later, the 1996 U.S. welfare reform substantially cut these programs.

In a discussion of the future political outcomes of an intellectually stratified society, they stated that they "fear that a new kind of conservatism is becoming the dominant ideology of the affluent - not in the social tradition of an Edmund Burke or in the economic tradition of an Adam Smith but 'conservatism' along Latin American lines, where to be conservative has often meant doing whatever is necessary to preserve the mansions on the hills from the menace of the slums below" (p. 518). Moreover, they fear that an increasing welfare will create a "custodial state" in which "a high-tech and more lavish version of the Indian reservation for some substantial minority of the nation's population." They also predict increasing totalitarianism: "It is difficult to imagine the United States preserving its heritage of individualism, equal rights before the law, free people running their own lives, once it is accepted that a significant part of the population must be made permanent wards of the states" (p. 526).

Responses
Initially, The Bell Curve received a great deal of positive publicity, including cover stories in Newsweek ("the science behind [it] is overwhelmingly mainstream"), early publication (under protest by other writers and editors) by The New Republic by its editor-in-chief at the time Andrew Sullivan, and The New York Times Book Review (which suggested critics disliked its "appeal to sweet reason" and are "inclined to hang the defendants without a trial"). Early articles and editorials appeared in Time, The New York Times ("makes a strong case"), The New York Times Magazine, Forbes, the Wall Street Journal, and National Review. It received a respectful airing on such shows as Nightline, the MacNeil/Lehrer NewsHour, the McLaughlin Group, Think Tank, PrimeTime Live, and All Things Considered. The book sold over 500,000 copies in hardcover.

While the book's popularity was mostly propelled by its controversial claims regarding race and intelligence, both the accuracy of those claims and the qualifications of the authors soon came under attack in the media. Herrnstein died before the book was released, leaving Charles Murray to the public defense. Although Herrnstein was a psychologist, Murray is a conservative think tank analyst with a Ph.D. in political science. Murray has no credentials in psychometrics.

Some scholars have condemned the book. University of Oklahoma Assistant Professor of Anthropology Michael Nunley wrote:

"I believe this book is a fraud, that its authors must have known it was a fraud when they were writing it, and that Charles Murray must still know it's a fraud as he goes around defending it. [...] After careful reading, I cannot believe its authors were not acutely aware of [...] how they were distorting the material they did include."

Professor Leon Kamin, a longtime critic of cognitive ability tests, said the book was "a disservice to and abuse of science." Harvard University psychologist Howard Gardner, who teaches in the department of education, called the style of thought "scholarly brinkmanship":

"The authors seem to show the evidence and leave the implications for the reader to figure out; discussing scientific work on intelligence, they never quite say that intelligence is all important and tied to one's genes, yet they signal that this is their belief and that readers ought to embrace the same conclusions."

Economist and conservative writer Thomas Sowell criticized the book's conclusions about race and the malleablility of IQ, writing: "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... 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."

In its defense, fifty-two professors, most of them psychologists including researchers in the study of intelligence and related fields, signed an opinion statement titled "Mainstream Science on Intelligence" endorsing the views presented in The Bell Curve. The statement was written by psychologist Linda Gottfredson and published in The Wall Street Journal in 1994 and reprinted in the Intelligence. Some of the signers were cited as sources for Murray and Herrnstein's book.

American Psychological Association task force report
In response to the growing controversy surrounding The Bell Curve, the American Psychological Association's Board of Scientific Affairs established a special task force to publish an investigative report on the research presented in the book. The final report, titled Intelligence: Knowns and Unknowns, is available at an academic website. 

Some of the task force's findings supported or were consistent with statements from The Bell Curve. They agreed that:
 * IQ scores have high predictive validity for individual differences in school achievement.
 * IQ scores have predictive validity for adult occupational status, even when variables such as education and family background have been statistically controlled.
 * Individual differences in intelligence are substantially influenced by both genetics and environment.
 * There is little evidence to show that childhood diet influences intelligence except in cases of severe malnutrition.
 * There are no statistically significant differences between the IQ scores of males and females.

Regarding Murray and Herrnstein's claims about racial differences and genetics, the APA task force stated: "There is certainly no such support for a genetic interpretation... . It is sometimes suggested that the Black/ White differential in psychometric intelligence is partly due to genetic differences (Jensen, 1972). There is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis."

Regarding statements about other explanations for racial differences, the APA task force stated: "The differential between the mean intelligence test scores of Blacks and Whites (about one standard deviation, although it may be diminishing) does not result from any obvious biases in test construction and administration, nor does it simply reflect differences in socio-economic status. Explanations based on factors of caste and culture may be appropriate, but so far have little direct empirical support."

Regarding statements about any explanations for racial differences, the APA task force stated: "At present, no one knows what causes this differential."

The APA journal that published the statement, American Psychologist, subsequently published eleven critical responses in January 1997.

See also: the discussion of intelligence testing

Criticisms
It is doubtful whether any book in the entire history of psychology has been so extensively attacked as The Bell Curve. Perhaps the most prominent critic of The Bell Curve was the late Stephen Jay Gould, who in 1996 released a revised and expanded edition of his 1981 controversial book The Mismeasure of Man intended to refute many of The Bell Curve's claims regarding race and intelligence. Specifically, Gould argues that the current evidence showing heritability of IQ does not indicate a genetic origin to group differences in intelligence. Murray claims that Gould misstated his claims; for instance, Gould says Murray boils down intelligence to a single factor while Murray denies making such a claim.

The initial positive reception of The Bell Curve in media such as newspapers and television talk shows was troubling to critics such as economist Edward S. Herman and evolutionary biologist Joseph L. Graves who felt that it indicated a troubling acceptance of what Herman calls deterministic racist doctrines. Dennis M. Rutledge suggests that through soundbites of works like Jensen's famous study on the achievement gap, and Herrnstein and Murray's book The Bell Curve, the media "paints a picture of Blacks and other people of color as collective biological illiterates-- as not only intellectually unfit but evil and criminal as well." thus providing, he says "the logic and justification for those who would further disenfranchise and exclude racial and ethnic minorities."

But the second wave of reviews, which did not arrive until much later, was comprised of expert opinion in the relevant fields. It provided a belated substitute for the peer-review process to which Murray and Hernstein were originally unwilling to submit. Scholarly examination repeatedly demonstrated that the statements that form the very core of The Bell Curve’s arguments were either highly questionable or demonstrably false.

Flawed assumptions
Much of the criticism of The Bell Curve has focused on the potential for faults in the basic assumptions made at the beginning of the book. William J. Matthews and Stephen Jay Gould list four basic assumptions of The Bell Curve:
 * 1) Intelligence must be depictable as a single number.
 * 2) Intelligence must be capable of rank ordering people in a linear order.
 * 3) Intelligence must be primarily genetically based.
 * 4) Intelligence must be essentially immutable.

If any of these premises are false then their entire argument disintegrates (Gould, 1994). Similarly, in "Science" in the service of Racism C. Loring Brace writes that The Bell Curve makes six basic assumptions at the beginning of the book:


 * 1) Human Cognitive ability is a single general entity, depictable as a single number.
 * 2) Cognitive ability has a heritability of between 40 and 80 percent and is therefore primarily genetically based.
 * 3) IQ is essentially immutable, fixed over the course of a life span.
 * 4) IQ tests measure how "smart" or "intelligent" people are and are capable of rank ordering people in a linear order.
 * 5) IQ tests can measure this accurately.
 * 6) IQ tests are not biased with regard to race ethnic group or socioeconomic status.

Brace proceeds to argue that there are faults in every one of these assumptions. The Bank of Sweden Prize winning economist James Heckman writes that two assumptions made in the book are questionable:


 * 1) "g" accounts for correlation across test scores and performance in society.
 * 2) "g" cannot be manipulated.

Heckman writes that a reanalysis of the evidence used in The Bell Curve contradicts this story. The factors that explain wages receive different weights than the factors that explain test scores. More than "g" is required to explain either. Other factors besides "g" contribute to social performance, and they can be manipulated. Murray responded to a shorter version of Heckman's critique in an August 1995 letter exchange in Commentary magazine.

Flawed methodology
Michael Hout of the University of California at Berkeley, along with five colleagues, recalculated the effect of socioeconomic status, using the same variables as The Bell Curve but weighting them differently. They found that if IQ scores are corrected to eliminate the effect of education as Herrnstein and Murrays did, the ability of IQ to predict poverty can be made to look dramatically overstated by as much as 61 percent for whites and 74 percent for blacks. In other words, according to Hout et al., Herrnstein and Murray's finding, that IQ predicts poverty much better than socioeconomic status does, is substantially a result of the way they handled the statistics. In a paper titled A Reanalysis of The Bell Curve researchers found that Herrnstein and Murray's measure of parental socioeconomic status failed to capture the effects of important elements of family background (such as single-parent family structure at age 14). As a result, their analysis gave an exaggerated impression of the importance of IQ relative to parents` SES, and relative to family background.

In the book Intelligence, Genes, and Success: Scientists Respond to The Bell Curve a group of social scientists and statisticians analyzes the genetics-intelligence link, the concept of intelligence, the malleability of intelligence and the effects of education, the relationship between cognitive ability, wages and meritocracy, pathways to racial and ethnic inequalities in health, and the question of public policy. This work argues that much of the public response was polemic and failed to analyze the details of the science and validity of the statistical arguments underlying the book's conclusions.

William J. Matthews writes that part of The Bell Curve's analysis is based on the AFQT "which is not an IQ test but designed to predict performance of certain criterion variables". Nobel Prize in Economics winner James J. Heckman observed that the AQFT was designed only to predict success in military training schools and that most of these tests appear to be achievement tests rather than ability tests, measuring factual knowledge and not pure ability. He continues:
 * 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.

Contradictory findings


A recent paper in the Psychological Review, "Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved" presents a mechanism by which environmental effects on IQ may be magnified by feedback effects. This approach may provide a resolution of the contradiction between the viewpoint of The Bell Curve and its supporters, and the 'nurture' factors of IQ believed to exist by its critics. Janet Currie and Duncan Thomas presented evidence suggesting AFQT scores are likely better markers for family background than "intelligence" in a 1999 Study. "Herrnstein and Murray report that conditional on maternal 'intelligence' (AFQT scores), child test scores are little affected by variations in socio-economic status. Using the same data, we demonstrate their finding is very fragile." Charles R. Tittle, Thomas Rotolo found that the more written, IQ-like examinations are used as screening devices for occupational access, the stronger the relationship between IQ and income. Thus, rather than higher IQ leading to status attainment because it indicates skills needed in a modern society, IQ may reflect the same test-taking abilities used in artificial screening devices by which status groups protect their domains. Min-Hsiung Huang and Robert M. Hauser write that Herrnstein and Murray provide scant evidence of growth in cognitive sorting. Using data from the General Social Survey, they tested each of these hypotheses using a short verbal ability test which was administered to about 12,500 American adults between 1974 and 1994 the results provided no support for any of the trend hypotheses advanced by Herrnstein and Murray. One chart in The Bell Curve purports to show that people with IQs above 120 have become "rapidly more concentrated" in high-IQ occupations since 1940. But Robert Hauser and his colleague Min-Hsiung Huang retested the data and came up with estimates that fell “well below those of Herrnstein and Murray." They add that the data, properly used, "do not tell us anything except that selected, highly educated occupation groups have grown rapidly since 1940."

Fueling racism
Dennis M. Rutledge suggests that through soundbites of works like Jensen's famous study on the achievement gap, and Herrnstein and Murray's book The Bell Curve, the media "paints a picture of Blacks and other people of color as collective biological illiterates-- as not only intellectually unfit but evil and criminal as well." thus providing, he says "the logic and justification for those who would further disenfranchise and exclude racial and ethnic minorities." Many critics point out that 17 of the researchers whose work is referenced by the book are also contributors to Mankind Quarterly, a magazine with a slant towards racial eugenics. In his book The Bell Curve Wars: Race, Intelligence, and the Future of America Steven Fraser writes that by scrutinizing the footnotes and bibliography in The Bell Curve, readers can more easily recognize the project for what it is: a chilly synthesis of the work of disreputable race theorists and eccentric eugenicists. Since the book promulgated the idea that blacks were on average less intelligent than whites, some people fear that The Bell Curve can be used by extremists to justify genocide and hate crimes. Critics have noted much of the work referenced by the Bell Curve was funded by the Pioneer Fund, which aims to advance the scientific study of heredity and human differences, and has been accused of promoting scientific racism.

Responses to criticisms
Arthur Jensen, Professor of Educational Psychology, University of California, Berkeley, responds to Gould's criticisms in a paper titled The Debunking of Scientific Fossils and Straw Persons

According to Christopher Chabirs, the most common responses to The Bell Curve involve "phony controversies". Chabris writes that "the vast majority of those commenting on The Bell Curve" in books such as The Bell Curve Wars "have little or no scientific authority".

Author's follow-up
Murray responded to specific criticisms of the analysis of the practical importance of IQ compared to socio-economic status (Part II of The Bell Curve) in a 1998 book Income Inequality and IQ To circumvent criticisms surrounding their use of a statistical control for socioeconomic status (SES), Murray adopted a sibling design. Rather than statistically controlling for parental SES, Murray compared life outcome differences among full sibling pairs who met a number of criteria in which one member of the pair has an IQ in the "normal" range and the other siblings has an IQ in a higher or lower IQ category. According to Murray, this design controls for all aspects of family background (full siblings share the same family background, growing up together in the same home and the same community).

Miscellanea
From 1986 to 1989, Murray was given an annual grant by the conservative Bradley Foundation of $90,000, rising to $113,000 by 1991, and then to $163,000 following publication of The Bell Curve.

According to an ABC news report, the Pioneer Fund contributed $3.5 million to researchers cited in The Bell Curve, and almost half of the research cited to support the most controversial racial conclusions of the book was paid for by the Pioneer Fund. 

The Bell Curve provided the first discussion of the Flynn effect aimed at a general audience, and was the first work to refer to it by that name (pp. 307–09).

The band Moxy Fruvous alluded to the authors of The Bell Curve, Richard J. Herrnstein and Charles Murray, in their song "Kick In The Ass", albeit misattributing the book to only a single author: "that guy who wrote that book about the bell curve, kick in the ass, kick in the ass"

Arguments against The Bell Curve

 * "Flattening The Bell Curve" by Nicholas Lemann in Slate Magazine, Jan. 18, 1997
 * Fairness and Accuracy in Reporting, Racism Resurgent: How Media Let The Bell Curve's Pseudo-Science Define the Agenda on Race
 * Critical analysis of Bell Curve methodology
 * The Return of Determinism? The Pseudoscience of the Bell Curve
 * Nobel Prize-winner James Heckman, "Lessons from the Bell Curve", Journal of Political Economy, October 1995.
 * ''The Bell Curve Wars: Intelligence and the Future of America", Steven Frazer, editor
 * The Bell Curve Fallacy. A layman's argument against the Bell Curve

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