The Bell Curve: Intelligence and Class Structure in American Life (20 page)

As soon as a town grows larger than a few dozen households in size, it starts to develop neighborhoods. As towns become cities, this tendency becomes a reliable law of human communities. People seek out comfortable neighborhoods they can afford. For some people, this will mean looking for a particular kind of setting. Parents with young children typically want parks, good schools, and neighbors with young children. Single
people in their twenties and thirties making good money often gravitate toward upscale urban neighborhoods with lots of places to go and things to do.

The result is to produce neighborhoods with a high level of socio-economic partitioning. The factory worker seldom lives next door to the executive, and this was as true in 1900 as in the last years of the century. The wealthy people have always been the most mobile. But in the late twentieth century, the most mobile people are increasingly drawn from the cognitive elite. In thinking about these changes, we will focus on their implications for the way that the children of the cognitive elite are raised, for therein lies one of the main potential sources of trouble.

First, the urbanization of the nation has meant that a much smaller proportion of the population grows up in places where socioeconomic mixing occurs naturally. Given a small enough town, there are not enough elementary schools to segregate the children efficiently. The children of the local upper crust may live on the street with the large houses, but there are not enough of them to fill up a whole school. After elementary school, every child in the town goes to the same middle school and high school. Such towns now constitute a shrinking proportion of the population, however. As of 1990, 78 percent of the overall population lived in metropolitan areas.
²³

Cognitive segregation is also being intensified by failures of government in large cities. As urban school systems deteriorate, people with money relocate to rich suburbs because that is where the good public school systems are; if they remain in the city, they send their children to private schools, which are even more homogeneous. As crime rates rise, people with money relocate to suburbs where the crime rates are low, or they concentrate ever more densely within the safer parts of the city. As urban tax rates rise, the middle class flees, leaving behind even more starkly segregated poles of rich and poor.

Bright working-class youngsters mix with children of every other level of ability in elementary school, but they are increasingly likely to be drawn away to the more intellectually homogeneous high school courses, thence to college. Much of the cognitive talent that used to be in the working-class neighborhood is being whisked up and out of the community through an educational system that is increasingly driven by academic performance. Because of residential segregation, the children
of lawyers, physicians, college professors, engineers, and business executives tend to go to schools with each other’s children, and seldom with the children of cab drivers or assembly-line workers, let alone with the children of welfare recipients or the chronically unemployed. They may never go to school with children representative of the whole range of cognitive ability. This tendency is exacerbated by another force working in the background, genes.

Twenty years ago, one of us wrote a book that created a stir because it discussed the heritability of IQ and the relationship of intelligence to success in life, and foresaw a future in which socioeconomic status would increasingly be inherited. The logic of the argument was couched in a syllogism:

If differences in mental abilities are inherited, and

If success requires those abilities, and

If earnings and prestige depend on success,

Then social standing (which reflects earnings and prestige) will be based to some extent on inherited differences among people.
²⁴

As stated, the syllogism is not fearsome. If intelligence is only trivially a matter of genes and if success in life is only trivially a matter of intelligence, then success may be only trivially inherited.

In fact, IQ is substantially heritable. The state of knowledge does not permit a precise estimate, but half a century of work, now amounting to hundreds of empirical and theoretical studies, permits a broad conclusion that the genetic component of IQ is unlikely to be smaller than 40 percent or higher than 80 percent.
²⁵
The most unambiguous direct estimates, based on identical twins raised apart, produce some of the highest estimates of heritability.
²⁶
For purposes of this discussion, we will adopt a middling estimate of 60 percent heritability, which, by extension, means that IQ is about 40 percent a matter of environment. The balance of the evidence suggests that 60 percent may err on the low side.

Because IQ and genes has been such a sensitive topic, it is worth a short digression to give some idea of where these estimates come from and how trustworthy they are.

First, consider the question that heads this section, not its answer. What we want to know is how much of the variation in IQ in a
population—
the aggregated differences among the individuals
²⁷
—is due to variations in genetic endowments and how much is due to variations in environment. If all the population variation in IQ is due to variations in environment, then the
heritability
is 0;
²⁸
if half is due to environmental variations, it is .5; if none is due to environmental variations, it is 1.0. Heritability, in other words, is a ratio that ranges between 0 and 1 and measures the relative contribution of genes to the variation observed in a trait.
²⁹

Specialists have come up with dozens of procedures for estimating heritability. Nonspecialists need not concern themselves with nuts and bolts, but they may need to be reassured on a few basic points. First, the heritability of any trait can be estimated as long as its variation in a population can be measured. IQ meets that criterion handily. There are, in fact, no other human traits—physical or psychological—that provide as many good data for the estimation of heritability as the IQ. Second, heritability describes something about a population of people, not an individual. It makes no more sense to talk about the heritability of an individual’s IQ than it does to talk about his birthrate. A given individual’s IQ may have been greatly affected by his special circumstances even though IQ is substantially heritable in the population as a whole. Third, the heritability of a trait may change when the conditions producing variation change. If, one hundred years ago, the variations in exposure to education were greater than they are now (as is no doubt the case), and if education is one source of variation in IQ, then, other things equal, the heritability of IQ was lower then than it is now.

This last point is especially important in the modern societies, with their intense efforts to equalize opportunity. As a general rule, as
environments become more uniform, heritability rises.
When heritability rises, children resemble their parents more, and siblings increasingly resemble each other; in general, family members become more similar to each other and more different from people in other families. It is the central irony of egalitarianism: Uniformity in society makes the members of families more similar to each other and members of different families more different.

Now for the answer to the question, How much is IQ a matter of genes? Heritability is estimated from data on people with varying amounts of genetic overlap and varying amounts of shared environment. Broadly speaking, the estimates may be characterized as direct or indirect.
³⁰
Direct estimates are based on samples of blood relatives who were raised apart. Their genetic overlap can be estimated from basic genetic considerations. The direct methods assume that the correlations between them are due to the shared genes rather than shared environments because they do not, in fact, share environments, an assumption that is more or less plausible, given the particular conditions of the study. The purest of the direct comparisons is based on identical (monozygotic, MZ) twins reared apart, often not knowing of each other’s existence. Identical twins share all their genes, and if they have been raised apart since birth, then the only environment they shared was that in the womb. Except for the effects on their IQs of the shared uterine environment, their IQ correlation directly estimates heritability. The most modern study of identical twins reared in separate homes suggests a heritability for general intelligence between .75 and .80, a value near the top of the range found in the contemporary technical literature.
³¹
Other direct estimates use data on ordinary siblings who were raised apart or on parents and their adopted-away children. Usually, the heritability estimates from such data are lower but rarely below .4.
³²

Indirect methods compare the IQ correlations between people with different levels of shared genes growing up in comparable environments—siblings versus half-siblings or versus cousins, for example, or MZ twins versus fraternal (dizygotic, DZ) twins, or nonadoptive siblings versus adoptive siblings. The underlying idea is that, for example, if full siblings raised in the same home and half-siblings raised in the same home differ in their IQ correlations, it is because they differ in the proportion of genes they share: full siblings share about 50 percent of genes, half siblings about 25 percent. Similarly, if siblings raised in unshared environments and cousins raised in unshared environments differ in their IQ correlations, it is because of the differing degrees of genetic overlap between cousins and siblings and not because of differing environmental influences, which are unshared by definition. And so on. Fleshed out in some sort of statistical model, this idea makes it possible to estimate the heritability, but the modeling can get complex. Some studies use mixtures of direct and indirect methods.
³³

The technical literature is filled with varying estimates of the heritability
of IQ, owing to the varying models being used for estimation and to the varying sets of data. Some people seem eager to throw up their hands and declare, “No one knows (or can know) how heritable IQ is.” But that reaction is as unwarranted as it is hasty, if one is content, as we are, to accept a range of uncertainty about the heritability that specialists may find nerve-racking. We are content, in other words, to say that the heritability of IQ falls somewhere within a broad range and that, for purposes of our discussion, a value of .6 ±.2 does no violence to any of the competent and responsible recent estimates. The range of .4 to .8 includes virtually all recent (since 1980) estimates—competent, responsible, or otherwise.
³⁴

Recent studies have uncovered other salient facts about the way IQ scores depend on genes. They have found, for example, that the more general the measure of intelligence—the closer it is to g—the higher is the heritability.
³⁵
Also, the evidence seems to say that the heritability of IQ rises as one ages, all the way from early childhood to late adulthood.
³⁶
This means that the variation in IQ among, say, youths ages 18 to 22 is less dependent on genes than that among people ages 40 to 44.[
³⁷
] Most of the traditional estimates of heritability have been based on youngsters, which means that they are likely to underestimate the role of genes later in life.

Finally, and most surprisingly, the evidence is growing that whatever variation is left over for the environment to explain (i.e., 40 percent of the total variation, if the heritability of IQ is taken to be .6), relatively little can be traced to the shared environments created by families.
³⁸
It is, rather, a set of environmental influences, mostly unknown at present, that are experienced by individuals as individuals. The fact that family members resemble each other in intelligence in adulthood as much as they do is very largely explained by the genes they share rather than the family environment they shared as children. These findings suggest deep roots indeed for the cognitive stratification of society.

The heritability of IQ is substantial. In Chapters 2 and 3, we presented evidence that the relationship of cognitive ability to success in life is far from trivial. Inasmuch as the syllogism’s premises cannot be dismissed out of hand, neither can its conclusion that success in life will be based to some extent on inherited differences among people.
³⁹

Furthermore, a variety of other scientific findings leads us to conclude that the heritability of success is going to increase rather than diminish. Begin with the limits that heritability puts on the ability to manipulate intelligence, by imagining a United States that has magically made good on the contemporary ideal of equality. Every child in this imaginary America experiences exactly the same environmental effects, for good or ill, on his or her intelligence. How much intellectual variation would remain? If the heritability of IQ is .6, the standard deviation of IQ in our magical world of identical environments would be 11.6 instead of 15 (see the note for how this calculation is done)—smaller, but still leaving a great deal of variation in intellectual talent that could not be reduced further by mere equalization.
⁴⁰
As we noted earlier, when a society makes good on the ideal of letting every youngster have equal access to the things that allow latent cognitive ability to develop, it is in effect driving the environmental component of IQ variation closer and closer to nil.

The United States is still very far from this state of affairs at the extremes. If one thinks of babies growing up in slums with crack-addicted mothers, at one extreme, compared to children growing up in affluent, culturally rich homes with parents dedicated to squeezing every last IQ point out of them, then even a heritability of .6 leaves room for considerable change if the changes in environment are commensurably large. We take up the evidence on that issue in detail in Chapter 17, when we consider the many educational and social interventions that have attempted to raise IQ. But those are, by definition, the extremes, the two tails of the distribution of environments. Moving a child from an environment that is the very worst to the very best may make a big difference. In reality, what most interventions accomplish is to move children from awful environments to ones that are merely below average, and such changes are limited in their potential consequences when heritability so constrains the limits of environmental effects.
⁴¹