The introduction states six of the authors’ assumptions, which they claim to be “beyond significant technical dispute”:[6]
- There is such a difference as a general factor of cognitive ability on which human beings differ.
- All standardized tests of academic aptitude or achievement measure this general factor to some degree, but IQ tests expressly designed for that purpose measure it most accurately.
- IQ scores match, to a first degree, whatever it is that people mean when they use the word intelligent, or smart in 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.[5]
Part I. The Emergence of a Cognitive Elite[edit]
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.[5]
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.[5]
Part II. Cognitive Classes and Social Behavior[edit]
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.[5]
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 by the United States Department of Labor‘s Bureau of Labor Statistics tracking thousands of Americans starting in the 1980s. Only non-Hispanic whites are included in the analyses so as to demonstrate that the relationships between cognitive ability and social behavior are not driven by race or ethnicity.[5]
Herrnstein and Murray argue that intelligence is a better predictor of individuals’ outcomes than parental socioeconomic status. This argument is based on analyses where individuals’ IQ scores are shown to better predict their outcomes as adults than the socioeconomic status of their parents. Such results are reported for many outcomes, including poverty, dropping out of school, unemployment, marriage, divorce, illegitimacy, welfare dependency, criminal offending, and the probability of voting in elections.[5]
All participants in the NLSY took the Armed Services Vocational Aptitude Battery (ASVAB), a battery of ten tests taken by all who apply for entry into the armed services. (Some had taken an IQ test in high school, and the median correlation of the Armed Forces Qualification Test (AFQT) scores and those IQ test scores was .81). Participants were later evaluated for social and economic outcomes. In general, Herrnstein and Murray argued, IQ/AFQT scores were a better predictor of life outcomes than social class background. Similarly, after statistically controlling for differences in IQ, they argued that many outcome differences between racial-ethnic groups disappeared.
| IQ | <75 | 75–90 | 90–110 | 110–125 | >125 |
|---|---|---|---|---|---|
| US population distribution | 5 | 20 | 50 | 20 | 5 |
| Married by age 30 | 72 | 81 | 81 | 72 | 67 |
| Out of labor force more than 1 month out of year (men) | 22 | 19 | 15 | 14 | 10 |
| Unemployed more than 1 month out of year (men) | 12 | 10 | 7 | 7 | 2 |
| Divorced in 5 years | 21 | 22 | 23 | 15 | 9 |
| % of children w/ IQ in bottom decile (mothers) | 39 | 17 | 6 | 7 | – |
| Had an illegitimate baby (mothers) | 32 | 17 | 8 | 4 | 2 |
| Lives in poverty | 30 | 16 | 6 | 3 | 2 |
| Ever incarcerated (men) | 7 | 7 | 3 | 1 | 0 |
| Chronic welfare recipient (mothers) | 31 | 17 | 8 | 2 | 0 |
| High school dropout | 55 | 35 | 6 | 0.4 | 0 |
| Scored “Yes” on “Middle Class Values Index”[c 1] | 16 | 30 | 50 | 67 | 74 |
Values are the percentage of each IQ sub-population, among non-Hispanic whites only, fitting each descriptor.[8]
- ^ According to Herrnstein & Murray the “Middle Class Values Index” was intended “to identify among the NLSY population, in their young adulthood when the index was scored, those people who are getting along with their lives in ways that fit the middle-class stereotype.” To score “Yes” on the index, a NLSY subject had to meet all four of the following criteria:
- Received at least a high-school diploma
- Never interviewed while incarcerated
- Still married to one’s first spouse
- Men only: In the labor force, even if not employed
- Women only: Never gave birth outside of marriage
Part III. The National Context[edit]
This part of the book discusses ethnic differences in cognitive ability and social behavior. Herrnstein and Murray report that Asian Americans have a higher mean IQ than white Americans, who in turn outscore black Americans. The book argues that the black-white gap is not due to test bias, noting that IQ tests do not tend to underpredict the school or job performance of black individuals and that the gap is larger on apparently culturally neutral test items than on more culturally loaded items. The authors also note that adjusting for socioeconomic status does not eliminate the black-white IQ gap. However, they argue that the gap is narrowing.[5]
According to Herrnstein and Murray, the high heritability of IQ within races does not necessarily mean that the cause of differences between races is genetic. On the other hand, they discuss lines of evidence that have been used to support the thesis that the black-white gap is at least partly genetic, such as Spearman’s hypothesis. They also discuss possible environmental explanations of the gap, such as the observed generational increases in IQ, for which they coin the term Flynn effect. At the close of this discussion, they write:[5]
If the reader is now convinced that either the genetic or environmental explanation has won out to the exclusion of the other, we have not done a sufficiently good job of presenting one side or the other. It seems highly likely to us that both genes and environment have something to do with racial differences. What might the mix be? We are resolutely agnostic on that issue; as far as we can determine, the evidence does not yet justify an estimate.
The authors also stress that regardless of the causes of differences, people should be treated no differently.[5]
In Part III, the authors also repeat many of the analyses from Part II, but now compare whites to blacks and Hispanics in the NLSY dataset. They find that after controlling for IQ, many differences in social outcomes between races are diminished.[5]
The authors discuss the possibility that high birth rates among those with lower IQs may exert a downward pressure on the national distribution of cognitive ability. They argue that immigration may also have a similar effect.[5]
At the close of Part III, Herrnstein and Murray discuss the relation of IQ to social problems. Using the NLSY data, they argue that social problems are a monotonically decreasing function of IQ,[5] in other words at lower IQ scores the frequency of social problems increases.
Living Together[edit]
In this final chapter, the authors discuss the relevance of cognitive ability for understanding major social issues in America.[5]
Evidence for experimental attempts to raise intelligence is reviewed. The authors conclude that currently there are no means to boost intelligence by more than a modest degree.[5]
The authors criticize the “levelling” of general and secondary education and defend gifted education. They offer a critical overview of affirmative action policies in colleges and workplaces, arguing that their goal should be equality of opportunity rather than equal outcomes.[5]
Herrnstein and Murray offer a pessimistic portrait of America’s future. They predict that a cognitive elite will further isolate itself from the rest of society, while the quality of life deteriorates for those at the bottom of the cognitive scale. As an antidote to this prognosis, they offer a vision of society where differences in ability are recognized and everybody can have a valued place, stressing the role of local communities and clear moral rules that apply to everybody.[5]
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