CHAPTER ONE

The Black-White Test Score Gap



Edited by CHRISTOPHER JENCKS and MEREDITH PHILLIPS



Brookings Institution Press

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CHRISTOPHER JENCKS

MEREDITH PHILLIPS



The Black-White

Test Score Gap:

An Introduction

AFRICAN AMERICANS currently score lower than European Americans on vocabulary, reading, and mathematics tests, as well as on tests that claim to measure scholastic aptitude and intelligence. This gap appears before children enter kindergarten (figure 1-1), and it persists into adulthood. It has narrowed since 1970, but the typical American black still scores below 75 percent of American whites on most standardized tests. On some tests the typical American black scores below more than 85 percent of whites?

The black-white test score gap does not appear to be an inevitable fact of nature. It is true that the gap shrinks only a little when black and white children attend the same schools. It is also true that the gap shrinks only a little when black and white families have the same amount of schooling, the same income, and the same wealth. But despite endless speculation, no one has found genetic evidence indicating that blacks have less innate intellectual ability than whites. Thus while it is clear that eliminating the test score gap would require enormous effort by both blacks and whites and would probably take more than one generation, we believe it can be done.

This conviction rests mainly on three facts:

-- When black or mixed-race children are raised in white rather than black homes, their preadolescent test scores rise dramatically. Black adoptees' scores seem to fall in adolescence, but this is what we would expect if, as seems likely, their social and cultural environment comes to resemble that of other black adolescents and becomes less like that of the average white adolescent.

-- Even nonverbal IQ scores are sensitive to environmental change. Scores on nonverbal IQ tests have risen dramatically throughout the world since the 1930s. The average white scored higher on the Stanford-Binet in 1978 than 82 percent of whites who took the test in 1932. Such findings reinforce the implications of adoption studies: large environmental changes can have a large impact on test performance.

-- Black-white differences in academic achievement have also narrowed throughout the twentieth century. The best trend data come from the National Assessment of Educational Progress (NAEP), which has been testing seventeen-year-olds since 1971 and has repeated many of the same items year after year. Figure 1-2 shows that the black-white reading gap narrowed from 1.25 standard deviations in 1971 to 0.69 standard deviations in 1996. The math gap fell from 1.33 to 0.89 standard deviations. When Min-Hsiung Huang and Robert Hauser analyzed vocabulary scores for adults born between 1909 and 1969, the black-white gap also narrowed by half.

In a country as racially polarized as the United States, no single change taken in isolation could possibly eliminate the entire legacy of slavery and Jim Crow or usher in an era of full racial equality. But if racial equality is America's goal, reducing the black-white test score gap would probably do more to promote this goal than any other strategy that commands broad political support. Reducing the test score gap is probably both necessary and sufficient for substantially reducing racial inequality in educational attainment and earnings. Changes in education and earnings would in turn help reduce racial differences in crime, health, and family structure, although we do not know how large these effects would be.

This judgment contradicts Christopher Jencks and his colleagues' 1972 conclusion in Inequality that reducing cognitive inequality would not do much to reduce economic inequality. The reason is simple: the world has changed. In 1972 the best evidence about what happened to black workers with high test scores came from a study by Phillips Cutright, who had analyzed the 1964 earnings of men in their thirties who had taken the Armed Forces Qualification Test (AFQT) between 1949 and 1953. Overall, employed black men earned 57.5 percent of what whites earned. Among men with AFQT scores above the national average, black men earned 64.5 percent of what whites earned (figure 1-3). In such a world, eliminating racial differences in test performance did not seem likely to reduce the earnings gap very much.

Today's world is different. The best recent data on test scores and earnings come from the National Longitudinal Survey of Youth (NLSY), which gave the Armed Services Vocational Aptitude Battery to a national sample of young people in 1980. Among employed men who were 31 to 36 years old in 1993, blacks earned 67.5 percent of what whites earned, a modest but significant improvement over the situation in 1964. The big change occurred among blacks with test scores near or above the white average. Among men who scored between the 30th and 49th percentiles nationally, black earnings rose from 62 to 84 percent of the white average. Among men who scored above the 50th percentile, black earnings rose from 65 to 96 percent of the white average. In this new world, raising black workers' test scores looks far more important than it did in the 1960s.

Some skeptics have argued that scores on tests of this kind are really just proxies for family background. As we shall see, family background does affect test performance. But even when biological siblings are raised in the same family, their test scores hardly ever correlate more than 0.5. Among children who have been adopted, the correlation falls to around half that level. The claim that test scores are only a proxy for family background is therefore false. Furthermore, test score differences between siblings raised in the same family have sizable effects on their educational attainment and earnings. Thus while it is true that eliminating the black-white test score gap would not reduce the black-white earnings gap quite as much as figure 1-3 implies, the effect would surely be substantial.

Reducing the black-white test score gap would reduce racial disparities in educational attainment as well as in earnings. The nationwide High School and Beyond survey tested twelfth-graders in 1982 and followed them up in 1992, when they were in their late twenties. At the time of the followup only 13.3 percent of the blacks had earned a B.A., compared with 30 percent of the non-Hispanic whites. Many observers blame this disparity on black parents' inability to pay college bills, black students' lack of motivation, or the hostility that black students encounter on predominantly white college campuses. All these factors probably play some role. Nonetheless, figure 1-4 shows that when we compare blacks and whites with the same twelfth grade test scores, blacks are more likely than whites to complete college. Once we equalize test scores, High School and Beyond blacks' 16.7 point disadvantage in college graduation rates turns into a 5.9 point advantage.

Eliminating racial differences in test performance would also allow colleges, professional schools, and employers to phase out the racial preferences that have caused so much political trouble over the past generation. If selective colleges based their admission decisions solely on applicants' predicted college grades, their undergraduate enrollment would currently be 96 or 97 percent white and Asian. To avoid this, almost all selective colleges and professional schools admit African Americans and Hispanics whom they would not admit if they were white. Racial preferences of this kind are politically unpopular. If selective colleges could achieve racial diversity without making race an explicit factor in their admission decisions, blacks would do better in college and whites would nurse fewer political grudges.

Advocates of racial equality might be more willing to accept our argument that narrowing the test score gap is crucial to achieving their goals if they believed that narrowing the gap was really feasible. But pessimism about this has become almost universal. In the 1960s, racial egalitarians routinely blamed the test score gap on the combined effects of black poverty, racial segregation, and inadequate funding for black schools. That analysis implied obvious solutions: raise black children's family income, desegregate their schools, and equalize spending on schools that remain racially segregated. All these steps still look useful, but none has made as much difference as optimists expected in the early 1960s.

--The number of affluent black parents has grown substantially since the 1960s, but their children's test scores still lag far behind those of white children from equally affluent families. Income inequality between blacks and whites appears to play some role in the test score gap, but it is quite small.

--Most southern schools desegregated in the early 1970s, and southern black nine-year-olds' reading scores seem to have risen as a result. Even today, black third-graders in predominantly white schools read better than initially similar blacks who have attended predominantly black schools. But large racial differences in reading skills persist even in desegregated schools, and a school's racial mix does not seem to have much effect on changes in reading scores after sixth grade or on math scores at any age.

--Despite glaring economic inequalities between a few rich suburbs and nearby central cities, the average black child and the average white child now live in school districts that spend almost exactly the same amount per pupil. Black and white schools also have the same average number of teachers per pupil, the same pay scales, and teachers with almost the same amount of formal education and teaching experience. The most important resource difference between black and white schools seems to be that teachers in black schools have lower test scores than teachers in white schools. This is partly because black schools have more black teachers and partly because white teachers in black schools have unusually low scores.

For all these reasons, the number of people who think they know how to eliminate racial differences in test performance has shrunk steadily since the mid-1960s. While many people still think the traditional liberal remedies would help, few now believe they would suffice.

Demoralization among liberals has given new legitimacy to conservative explanations for the test score gap. From an empirical viewpoint, however, the traditional conservative explanations are no more appealing than their liberal counterparts. These explanations fall into three overlapping categories: the culture of poverty, the scarcity of two-parent black families, and genes.

--In the 1960s and 1970s, many conservatives blamed blacks' problems on a culture of poverty that rejected school achievement, the work ethic, and the two-parent family in favor of instant gratification and episodic violence. In the 1980s, conservatives (as well as some liberals) characterized the "black underclass" in similar terms. But this description only fits a tiny fraction of the black population. It certainly cannot explain why children from affluent black families have much lower test scores than their white counterparts.

--Conservatives invoke the decline of the family to explain social problems almost as frequently as liberals invoke poverty. But once we control a mother's family background, test scores, and years of schooling, whether she is married has even less effect on her children's test scores than whether she is poor.

--Scientists have not yet identified most of the genes that affect test performance, so we have no direct genetic evidence regarding innate cognitive differences between blacks and whites. But we have accumulated a fair amount of indirect evidence since 1970. Most of it suggests that whether children live in a "black" or "white" environment has far more impact on their test performance than the number of Africans or Europeans in their family tree.

Taken as a whole, then, what we have characterized as the "traditional" explanations for the black-white test score gap do not take us very far. This has led some people to dismiss the gap as unimportant, arguing that the tests are culturally biased and do not measure skills that matter in the real world. Few scholars who spend time looking at quantitative data accept either of these arguments, so they have had to look for new explanations of the gap. These new explanations can mostly be grouped under two overlapping headings: culture and schooling.

In the late 1960s and early 1970s, many blacks and some whites dismissed cultural explanations of the test score gap as an effort to put down blacks for not thinking and acting like upper-middle-class whites. Since then, cultural explanations have enjoyed a slow but steady revival. In 1978 the Nigerian anthropologist John Ogbu suggested that caste-like minorities throughout the world tended to do poorly in school, even when they were visually indistinguishable from the majority. Later, Ogbu argued that because blacks had such limited opportunities in America, they developed an "oppositional" culture that equated academic success with "acting white." By linking black culture directly to oppression, Ogbu made it much easier for liberals to talk about cultural differences. Jeff Howard and Ray Hammond added another important strand to this argument when they suggested that academic competence developed partly through competition, and that "rumors of inferiority" made blacks reluctant to compete academically. More recently, Claude Steele has argued that people of all races avoid situations in which they expect others to have negative stereotypes about them, even when they know that the stereotype does not apply. According to Steele, many black students "disidentify" with school because constructing a personal identity based on academic competence entails a commitment to dealing with such stereotypes on a daily basis.

Social scientists' thinking about "school effects" has also changed since the late 1960s. The 1966 Coleman Report and subsequent "production function" studies convinced most economists and quantitative sociologists that school resources had little impact on achievement. Since 1990, however, new statistical methods, new data, and a handful of genuine experiments have suggested that additional resources may in fact have sizable effects on student achievement. The idea that resources matter cannot in itself explain the black-white achievement gap, because most school resources are now fairly equally distributed between blacks and whites. But certain crucial resources, such as teachers with high test scores, are still unequally distributed. And other resources, such as small classes and teachers with high expectations, may help blacks more than whites. The idea that resources matter also suggests that "compensatory" spending on black schools could be valuable, at least if the money were used to cut class size and implement policies that have been shown to help.

This book tries to bring together recent evidence on some of the most controversial and puzzling aspects of the test score debate. Section I examines the role of test bias, heredity, and family background in the black-white gap. Section II looks at how and why the gap has changed over the past generation. Section III examines educational, psychological, and cultural explanations for the gap. Section IV analyzes some of the educational and economic consequences of the gap. The book concludes with a commentary by William Julius Wilson. The rest of the introduction summarizes the book's main findings and then discusses some of their implications.

Test Bias

Many blacks and some whites believe that all cognitive tests are racially biased. In chapter 2 Christopher Jencks discusses five possible varieties of racial bias in testing. He concludes that two of the five constitute serious problems and that three are probably of minor importance.

Labeling Bias

What Jencks calls "labeling bias" arises when a test claims to measure one thing but really measures something else. This is a major problem when tests claim to measure either intelligence or aptitude, because these terms are widely used to describe innate "potential" as well as developed abilities. The notion that intelligence and aptitude are innate seems to be especially salient in discussions of racial differences. Thus, the statement that "blacks are less intelligent than whites" is widely understood as a statement about innate differences. Yet almost all psychologists now agree that intelligence tests measure developed rather than innate abilities, and that people's developed abilities depend on their environment as well as their genes. Even psychologists who believe that racial differences in test performance are to some extent innate agree that intelligence tests overstate the difference one would observe if blacks and whites grew up in identical environments. Intelligence tests therefore constitute a racially biased estimate of innate ability, which is what nonpsychologists often mean by the word "intelligence." Test designers cannot eliminate this bias by changing the content of intelligence tests. The only way to eliminate it is to change the tests' labels so as to emphasize the fact that they measure developed rather than innate skills and abilities.

Content Bias

"Content bias" arises when a test contains questions that favor one group over another. Suppose, for example, that black and white children spoke mutually unintelligible versions of English. A test given in white English would then underestimate black children's skills and vice versa. This kind of content bias does not appear to be a significant problem for the tests discussed in this book. If one takes a standard vocabulary test and winnows out words with unusually large black-white differences, for example, the black-white gap does not shrink much. Likewise, if one compares black children to slightly younger white children, blacks and whites find the same words easy and difficult. Nor is the black-white gap on tests that measure familiarity with the content of American culture consistently larger than the gap on nonverbal tests that do not measure familiarity with any particular culture. Because the racial gap in children's test performance is not confined to items that measure exposure to white language, culture, or behavior but is dramatically reduced when black children are raised in white homes, Jencks suggests that it may reflect differences in the way blacks and whites are taught to deal with what they do not know and in the emphasis they put on learning new cognitive skills.

Methodological Bias

Methodological bias arises when we assess mastery of some skill or body of information in a way that underestimates the competence of one group relative to another. Methodological bias would be important if, say, having black rather than white testers changed the relative standing of black and white test takers. That does not appear to be the case. There is some evidence that describing a test in different ways can affect different groups' relative performance, but we do not yet know how general this is.

Prediction Bias

A generation ago many egalitarians argued that using the SAT to screen applicants for selective colleges was unfair to blacks because tests of this kind underestimated black applicants' academic potential. For most colleges, academic potential means undergraduate grades. Almost all colleges have found that when they compare black and white undergraduates who enter with the same SAT scores, blacks earn lower grades than whites, not just in their first year but throughout their college careers. Likewise, when firms compare black and white workers with the same test scores, blacks usually get slightly lower ratings from their supervisors and also do a little worse on more objective measures of job performance. In psychological parlance, this means that tests like the SAT do not suffer from "prediction bias."

Selection System Bias

The test score gap between black and white job applicants has traditionally averaged about one standard deviation. When employers do not screen workers, the performance gap is likely to be much smaller--typically more like two-fifths of a standard deviation. The reason for this discrepancy is not that blacks perform better than whites with the same test scores. The reason is that test scores explain only 10 to 20 percent of the variation in job performance, and blacks are far less disadvantaged on the noncognitive determinants of job performance than on the cognitive ones.

Because blacks perform no better on the job than whites with similar scores, many people assume that using tests to select workers is racially fair. But if racial fairness means that blacks and whites who could do a job equally well must have an equal chance of getting the job, a selection system that emphasizes test scores is almost always unfair to most blacks (and to everyone else with low test scores). Imagine a company that has 600 applicants for 100 openings. Half the applicants are black and half are white. If the firm hires all applicants as temporary workers and retains those who perform best on the job, and if the performance gap between blacks and whites averages 0.4 standard deviations, about 36 blacks will get permanent jobs. If the firm selects the 100 applicants with the highest scores, about 13 blacks will get permanent jobs. Jencks argues that the first outcome should be our yardstick for defining racial fairness. Using this yardstick, the second system is clearly biased against blacks. In effect, Jencks says, the second system forces blacks to pay for the fact that social scientists have unusually good measures of a trait on which blacks are unusually disadvantaged.

The Heredity-Environment Controversy

When the U.S. Army launched the world's first large-scale mental testing program in 1917, it found that whites scored substantially higher than blacks. Biological determinists immediately cited these findings as evidence that whites had more innate ability than blacks, but cultural determinists quickly challenged this interpretation. By the late 1930s most social scientists seem to have been convinced that either genetic or cultural factors could explain the gap. Neither side had a convincing way of separating the effects of heredity from the effects of culture, so the debate was an empirical standoff.

After 1945 the horrors of the Holocaust made all genetic explanations of human differences politically suspect. Once the U.S. Supreme Court declared de jure racial segregation unconstitutional in 1954, genetic explanations of racial differences became doubly suspect because they were identified with southern resistance to desegregation. As a result, environmentalism remained hegemonic throughout the 1960s. Then in 1969 Arthur Jensen published an article in the Harvard Educational Review arguing that educational programs for disadvantaged children initiated as part of the War on Poverty had failed, and that the black-white test score gap probably had a substantial genetic component? Jensen's argument went roughly as follows:

--Most of the variation in white IQ scores is genetic.

--No one has advanced a plausible environmental explanation for the black-white gap.

--Therefore it is more reasonable to assume that part of the black-white gap is genetic than to assume it is entirely environmental.

Jensen's article created such a furor that psychologists once again began looking for evidence that bore directly on the question of whether racial differences in test performance were partly innate. Richard Nisbett reviews their findings in chapter 3.

Two small studies have tried to compare genetically similar children raised in black and white families. Elsie Moore found that black children adopted by white parents had IQ scores 13.5 points higher than black children adopted by black parents. Lee Willerman and his colleagues compared children with a black mother and a white father to children with a white mother and a black father. The cleanest comparison is for mixed-race children who lived only with their mother. Mixed-race children who lived with a white mother scored 11 points higher than mixed-race children who lived with a black mother. Since the black-white IQ gap averaged about 15 points at the time these two studies were done, they imply that about four-fifths of that gap was traceable to family-related factors (including schools and neighborhoods).

A better-known study dealt with black and mixed-race children adopted by white parents in Minnesota. The mixed-race children were adopted earlier in life and had higher IQ scores than the children with two black parents. When the 29 black children were first tested, they scored at least ten points higher than the norm for black children, presumably because they had more favorable home environments than most black children. When these children were retested in their late teens or twenties, their IQ scores had dropped and were no longer very different from those of Northern blacks raised in black families. The most obvious explanation for this drop is that the adoptees had moved out of their white adoptive parents' homes into less favorable environments. But because the study did not cover black or mixed-race children adopted by black parents, it does not seem to us to provide strong evidence on either side of the heredity-environment debate.

Racially Mixed Children

Race is not a well-defined biological category. It is a social category, whose biological correlates vary geographically and historically. America has traditionally classified people as black using the "one drop" rule, under which anyone with known black ancestors is black. As a result, people are often treated as black even though they have a lot of European ancestors. If blacks with a lot of European ancestors had the same test scores as those with no European ancestors, we could safely conclude that the black-white test score gap was a by-product of social classification rather than heredity. But when we find that light-skinned blacks score higher than dark-skinned blacks, we cannot rule out the possibility that this difference is environmental. Light skin has traditionally been a social asset for black Americans, and the correlation between light skin and test performance could reflect this fact. To get around this problem, we need less visible genetic markers. Two studies have used blood markers to estimate the percentage of Europeans in a black child's family tree. Neither study found a correlation between the number of "European" blood markers and IQ.

Although racially mixed children are culturally black in America, and are almost always raised by black parents in black communities, this is not true everywhere. Klans Eyferth studied the illegitimate children of black and white soldiers stationed in Germany as part of the army of occupation after World War II. All these children were raised by their German mothers. There was considerable prejudice against blacks in Germany at the time, and any child of a German mother who looked black was also presumed to be illegitimate, which carried additional stigma. But mixed-race German children did not attend predominantly black schools, live in black neighborhoods, or (presumably) have predominantly black (or mixed-race) friends. When Eyferth gave these children a German version of the Wechsler IQ test, children with black fathers and white fathers had almost identical scores.

Taken in isolation, none of these studies would carry much weight. The samples are small, and the comparisons could be distorted by unmeasured genetic or environmental influences. But Nisbett argues that their consistency gives them far more weight that they would have if taken one by one. We agree. We read these studies as supporting three tentative conclusions:

--When "black" genes are not visible to the naked eye and are not associated with membership in a black community, they do not have much effect on young children's test scores.

--Growing up in an African-American rather than a European-American family substantially reduces a young child's test performance.

--When black Americans raised in white families reach adolescence, their test scores fall.

These studies do not prove that blacks and whites would have exactly the same test scores if they were raised in the same environment and treated the same way. But we find it hard to see how anyone reading these studies with an open mind could conclude that innate ability played a large role in the black-white gap.

Effects of Family Background

Chapter 4, by Meredith Phillips, Jeanne Brooks-Gunn, Greg Duncan, Pamela Klebanov, and Jonathan Crane tries to estimate the effect of specific family characteristics on young children's test scores. This is not easy. Hundreds of different family characteristics correlate with children's test performance. Disentangling their effects is a statistical nightmare. Almost any family characteristic can also serve as a proxy for a child's genes. We know, for example, that a mother's genes affect her test scores and that her test scores affect her educational attainment. Thus when we compare children whose mothers finished college to children whose mothers only finished high school, the two groups' vocabulary scores can differ for genetic as well as environmental reasons. Even when a child is adopted, moreover, the way the adoptive parents treat the child may depend on the child's genes. Parents read more to children who seem to enjoy it, for example, and whether children enjoy being read to may well depend partly on their genes.

The best solution to such problems is to conduct experiments. In the 1970s, for example, the federal government conducted a series of "negative income tax" experiments that increased the cash income of randomly selected low-income families. These experiments did not last very long, the samples getting any given "treatment" were small, and the results were poorly reported, so it is hard to know exactly what happened. Short-term income increases did not have statistically reliable effects on low-income children's test scores, but that does not mean their true effect was zero. As far as we know, these are the only randomized experiments that have altered families' socioeconomic characteristics and then measured the effect on children's test scores.

In theory, we can also separate the effects of parents' socioeconomic status from the effects of their genes by studying adopted children. But because adoption agencies try to screen out "unsuitable" parents, the range of environments in adoptive homes is usually restricted. The adoptive samples for which we have data are also small. Thus while parental SES does not predict adopted children's IQ scores as well as it predicts natural children's IQ scores, the data on adopted children are not likely to persuade skeptics.

Anyone who wants to estimate the impact of specific family characteristics on children's test scores must therefore rely heavily on surveys of children raised by their natural parents. The best source of such data is the National Longitudinal Survey of Youth (CNLSY) that Phillips and her colleagues use in chapter 4. Black five- and six-year-olds in their sample scored about 16 points (one standard deviation) below whites on the Peabody Picture Vocabulary Test (PPVT). Traditional measures of educational and economic inequality do not explain much of this gap. Measures of a mother's socioeconomic position when she was growing up and measures of her current parenting practices explain considerably more.

Parental Schooling

Early in the twentieth century white parents typically completed two or three more years of school than blacks. By 1991 the gap between black and white mothers with children in first grade had fallen to 0.8 years. Many observers have suggested that this change played a significant role in reducing the black-white gap in children's test scores. But if parental schooling correlates with children's test scores mainly because it is a proxy for parental genes, changing the distribution of schooling will not affect the distribution of test scores, either for individuals or for groups.

When Phillips and her colleagues control a mother's family background, the estimated effect of an extra year of school on her child's PPVT score falls from 1.73 to 1.15 points. When they also control the mother's AFQT score (their proxy for her cognitive genotype), the effect falls to somewhere between 0.5 and 0.6 points. This suggests that a two-year reduction in the black-white education gap among mothers would cut the PPVT gap by about a point. Of course, if the schooling gap narrowed because black and white parents had grown up in more similar homes or had more similar test scores, the predicted reduction in the PPVT gap between black and white children would be larger. The CNLSY suggests that cutting the schooling gap between black and white fathers would have a smaller effect than cutting the gap between black and white mothers. But this may not be true for older children, for whom the effects of mother's and father's education are roughly equal.

Income Effects

White CNLSY parents reported 73 percent more income than their black counterparts. When Phillips and her colleagues compared black and white children whose families had had the same average annual income since the child was born, the PPVT gap narrowed by 2.4 points. But once again it does not follow that raising all black families' incomes by 73 percent would raise their children's PPVT scores by 2.4 points. To estimate the effect of increasing black parents' incomes without changing the traits that cause the current income gap, we need to know how much parental income affects children's test scores when we compare parents with the same family background, test scores, and schooling. These controls cut the estimated effect of parental income on PPVT scores by about three-fifths. Even this estimate is probably on the high side, because it does not control either the father's test scores or his family background. Thus, the CNLSY suggests that eliminating black-white income differences would cut the PPVT gap by less than 1 point. Eliminating the causes of the black-white income gap might, of course, have a much larger effect. Racial disparities in parental wealth have almost no effect on children's test scores once Phillips and her colleagues control income, schooling, and the mother's test scores.

Single-Parent Families

Once Phillips and her colleagues hold constant the mother's family background, educational attainment, and test scores, children who have grown up in an intact family score no higher on the PPVT than children from single-parent families. Other studies find slightly larger effects, but the effects are never large enough to be of any substantive importance.

Parenting Strategies

Knowing parents' education and income tells us something about how they raise their children, but not much. The CNLSY tried to measure parenting practices directly, both by asking parents what they did and by asking interviewers how mothers treated their children during the interview. Parenting practices appear to have a sizable impact on children's test scores. Even with parental education, family income, and the mother's AFQT scores controlled, racial differences in parenting practices account for between a fifth and a quarter of the racial gap on the PPVT. This suggests that changes in parenting practices might do more to reduce the black-white test score gap than changes in parents' educational attainment or income. We cannot be sure how large these effects would be, however, because the way parents treat their children is a proxy for all kinds of unmeasured parental characteristics, as well as for the child's own genes.

Grandparents

Upwardly mobile parents often raise their children the way they themselves were raised. Phillips and her colleagues find that racial differences in parenting practices are partly traceable to the fact that even when black and white parents have the same test scores, educational attainment, income, wealth, and number of children, black parents are likely to have grown up in less advantaged households. Phillips and her colleagues also find that this can lower black children's test scores. In other words, it can take more than one generation for successful families to adopt the "middle-class" parenting practices that seem most likely to increase children's cognitive skills.