Adding education. Among high school graduates (no more, no less) in the NLSY, a plot of the results of the standard analysis looks visually identical to the one presented for the entire sample, but the sample of low-birth-weight babies was so small that the results do not reach statistical significance. Among the college graduates, low-birth-weight babies were so rare (only six out of 277 births to the white college sample) that a multivariate analysis produced no interpretable results. We do not know whether it is the education itself, or the self-selection that goes into having more education, that is responsible for their low incidence of underweight babies.
Infant Mortality
Though we have not been able to find any studies of cognitive ability and infant mortality, it is not hard to think of a rationale linking them. Many things can go wrong with a baby, and parents have to exercise both watchfulness and judgment. It takes more than love to childproof a house effectively; it also takes knowledge and foresight. It takes intelligence to decide that an apparently ordinary bout of diarrhea has gone on long enough to make dehydration a danger; and so on. Nor is simple knowledge enough. As pediatricians can attest, it may not be enough to tell new parents that infants often spike a high fever, that such episodes do not necessarily require a trip to the hospital, but that they require careful attention lest such a routine fever become life threatening. Good parental judgment remains vital. For that matter, the problem facing pediatricians dealing with children of less competent parents is even more basic than getting them to apply good judgment: It is to get such mothers to administer the medication that the doctor has provided.
This rationale is consistent with the link that has been found between education and infant mortality. In a study of all births registered in California in 1978, for example, infant deaths per 1,000 to white women numbered 12.2 for women with less than twelve years of education, 8.3 for those with twelve years, and 6.3 for women with thirteen or more years of education, and the role of education remained significant after controlling for birth order, age of the mother, and marital status.56
We have been unable to identify any study that uses tested IQ as an explanatory factor, and, with such a rare event as infant mortality, even the NLSY cannot answer our questions satisfactorily. The results certainly suggest that the questions are worth taking seriously. As of the 1990 survey, the NLSY recorded forty-two deaths among children born to white women with known IQ. Some of these deaths were presumably caused by severe medical problems at birth and occurred in a hospital where the mother’s behavior was irrelevant.57 For infants who died between the second and twelfth month (the closest we can come to defining “after the baby had left the hospital”), the mothers of the surviving children tested six points higher in IQ than the mothers of the deceased babies. (The difference for mothers of children who died in the first month was not quite three points and for the mothers of children who were older than 1 year old when they died, virtually zero.) The samples here are too small to analyze in conjunction with socioeconomic status and other variables.
POVERTY THROUGHOUT EARLY CHILDHOOD
In Chapter 5, we described how the high-visibility policy issue of children in poverty can be better understood when the mother’s IQ is brought into the picture. Here, we focus more specifically on the poverty in the early years of a child’s life, when it appears to be an especially important factor (independent of other variables) in affecting the child’s development.58 The variable is much more stringent than simply experiencing poverty at some point in childhood. Rather, we ask about the mothers of children who lived under the poverty line throughout their first three years of life, comparing them with mothers who were not in poverty at any time during the child’s first three years. The standard analysis is shown in the figure below. There are few other analyses in Part II that show such a steep effect for both intelligence and SES. If the mother has even an average intelligence and average socioeconomic background, the odds of a white child’s living in poverty for his or her first three years were under 5 percent. If either of those conditions fell below average, the odds increased steeply.
A white mother’s IQ and socioeconomic background each has a large independent effect on her child’s chances of spending the first three years of life in poverty
Note: For computing the plot, age and either SES (for the black curve) or IQ (for the gray curve) were set at their mean values.
The Role of Preexisting Poverty
When we ask whether the mother was in poverty in the year prior to birth, it turns out that a substantial amount of the effect we attribute to socioeconomic background in the figure really reflects whether the mother was already in poverty when the child was born. If you want to know whether a child will spend his first three years in poverty, the single most useful piece of information is whether the mother was already living under the poverty line when he was born. Nevertheless, adding poverty to the equation does not diminish a large independent role for cognitive ability. A child born to a white mother who was living under the poverty line but was of average intelligence had almost a 49 percent chance of living his first three years in poverty. This is an extraordinarily high chance of living in poverty for American whites as a whole. But if the same woman were at the 2d centile of intelligence, the odds rose to 89 percent; if she were at the 98th centile, they dropped to 10 percent.59 The changes in the odds were proportionately large for women who were not living in poverty when the child was born.
The Role of Education
For children of women with a high school diploma (no more, no less), the relationships of IQ and socioeconomic background to the odds that a child would live in poverty are the same as shown in the figure above—almost equally important, with socioeconomic background fractionally more so—except that the odds are a little lower than for the whole sample (the highest percentages, for mothers two standard deviations below the mean, are in the high 20s, instead of the mid-30s). As this implies, the highest incidence of childhood poverty occurs among women who dropped out of school Among the white college sample (a bachelor’s degree, no more and no less), there was nothing to analyze; only one child of such mothers had lived his first three years in poverty.
IQ AND THE HOME ENVIRONMENT FOR CHILD DEVELOPMENT
In 1986, 1988, and 1990, the NLSY conducted special supplementary surveys of the children and mothers in the sample. The children were given tests of mental, emotional, and physical development, to which we shall turn presently. The mothers were questioned about their children’s development and their rearing practices. The home situation was directly observed. The survey instruments were based on the so-called HOME (Home Observation for Measurement of the Environment) index.60
Dozens of questions and observations go into creating the summary measures, many of them interesting in themselves. Children of the brightest mothers who also tend to be the best educated and the most affluent) have a big advantage in many ways, especially on such behaviors as reading to the child. On other indicators that are less critical in themselves, but indirectly suggest how the child is being raised, children with smarter mothers also do better. For example, mothers in the top cognitive classes use physical punishment less often (though they agree in principle that physical punishment can be an appropriate response), and the television set is off more of the time in the homes of the top cognitive classes.
Treating the HOME index as a continuous scale running from “very bad” to “very good” home environments, the advantages of white children with smarter mothers were clean The average child of a Class V woman lived in a home at the 32d percentile of home environment, while the home of the average child of a Class I woman was at the 76th percentile. The gradations for the three intervening classes were regular as well.61 Overall, the correlation of the HOME index with IQ for white mothers was +.24, statistically significant but hardly overpowering.
In trying to identify children at risk, this way of looking at the relationship is not necessarily the most reveal
ing. We are willing to assume that a child growing up in a home at the 90th centile on the HOME index has a “better” environment than one growing up at the 50th. Perhaps the difference between a terrific home environment and a merely average one helps produce children who are at the high end on various personality and achievement measures. But it does not necessarily follow that the child in the home at the 50th centile is that much more at risk for the worst outcomes of malparenting than the child at the 90th centile. Both common sense and much of the scholarly work on child development suggest that children are resilient in the face of moderate disadvantages and obstacles and, on the other hand, that parents are frustratingly unable to fine-tune good results for their children.
But resilience has its limits. Children coming from the least nurturing, most punishing environments are indeed at risk. We will therefore focus throughout this section on children who are in the bottom 10 percent on various measures of their homes.62
Which White Children Grow Up in the Worst Homes?
Cognitive Class of the Mother Percentage of Children Growing Up in Homes in the Bottom Decile of the HOME Index
I Very bright 0
II Bright 2
III Normal 6
IV Dull 11
V Very dull 24
All whites 6
In the case of the HOME index, the percentages of white children of mothers in the different cognitive classes who are growing up in homes that scored at the bottom are displayed in the table above. It was extremely rare for children of women in the top cognitive classes to grow up in these “worst homes” and quite uncommon for children of women throughout the top three-fourths of the IQ distribution. Only in the bottom cognitive classes did the proportion of such children grow, and then the proportions rose rapidly. Nearly one out of four of the children of the dullest mothers was growing up in a home that also ranked in the bottom decile on the HOME index.63
The Role of Socioeconomic Background
The usual assumption about maternal behavior is that a woman’s socioeconomic status is crucial—that she passes on to her children the benefits or disadvantages of her own family background. The figure below summarizes the standard analysis comparing SES and IQ.
A white mother’s IQ is more important than her socioeconomic background in predicting the worst home environments
Note: For computing the plot, age and either SES (for the black curve) or IQ (for the gray curve) were set at their mean values. Additional independent variables were used to control for the test year and the age of the children.
Both factors play a significant role, but once again it is worse (at least for the white NLSY population) to have a mother with a low IQ than one from a low socioeconomic background. Given just an average IQ for the mother, even a mother at the 2d centile on socioeconomic background had less than a 10 percent chance of providing one of the “worst homes” for her children. But even with average socioeconomic background, a mother at the 2d centile of intelligence had almost a 17 percent chance of providing one of these “worst homes.”
The Role of Poverty and Welfare
Many of the problems experienced by poor children are usually attributed in both public dialogue and academic writings to poverty itself.64 The reasons for this widely assumed link between poverty and developmental problems are harder to spell out than you might think. To repeat a point that must always be kept in mind when thinking about poverty: Most of the world’s children throughout history have grown up poor, with “poverty” meaning material deprivation far more severe than the meaning of “below the poverty line” in today’s America. Many of the disadvantages today’s children experience are not the poverty itself but the contemporary correlates of poverty: being without a father, for example, or living in high-crime neighborhoods. Today, high proportions of poor children experience these correlates; fifty years ago, comparatively few poor children did.
But there are reasons to think that the HOME index might be influenced by poverty. Reading to children is a good thing to do, for example, and raises the HOME score, but children’s books are expensive. It is easier to have books in the house if you can afford to buy them than if you have to trek to the library—perhaps quite far from home—to get them. Similar comments apply to many of the indicators on the HOME index that do not require wealth but could be affected by very low income. We therefore explored how the HOME index was related to the mother’s poverty or welfare recipiency in the calendar year before the HOME score was obtained.65
Poverty proved to be important, with “being in a state of poverty” raising the odds of being in the worst decile of the HOME index from 4 percent to 11 percent, given a mother of average IQ and socioeconomic status.66 But adding poverty to the equation did not diminish the independent role of cognitive ability. For example, if the mother had very low IQ (the 2d centile) and was in poverty, the odds of being in the worst decile on the HOME index jumped from 11 percent to 26 percent. Generally, adding poverty to the analysis replaced the impact of the mother’s socioeconomic background, not of her intelligence.
Then we turn to welfare. The hypothesis is that going on welfare signifies personality characteristics other than IQ that are likely to make the home environment deficient—irresponsibility, immaturity, or lack of initiative, for example. Therefore, the worst homes on the HOME index will also tend to be welfare homes. This hypothesis too is borne out by the data: Welfare recipiency was a slightly more powerful predictor of being in a “worse home” than poverty—but it had as little effect on the independent role of IQ.
In trying to decide among competing explanations, the simplest thing to do is to enter both poverty and welfare in the analysis and see which wins out. We summarize the outcome by first considering a child whose mother is of average intelligence and socioeconomic background. If his mother is either poor or on welfare (but not both), the odds of having a terrible home environment (bottom decile on the HOME index) are 8 or 9 percent. If the mother has an IQ of 70, the odds shoot up to 18 to 21 percent. If the mother has very low intelligence, is poor, and is also on welfare, the odds rise further, to 34 percent. A table with some of the basic permutations is given in the note.67
Still, many of the causal issues remain unresolved. The task for scholars is to specify what it is about poverty that leads to the outcomes associated with it. With the data at hand, we cannot go much further in distinguishing between the effects of lack of money and the effects of other things that “being in poverty” signifies. In particular, the way that poverty and welfare interact in producing a poor home environment provides many hints that need to be followed up.
What can be said unequivocally is that low income as such does not prevent children from being raised in a stimulating, nurturing environment. Such is the story of the regression coefficients, and a conclusion that accords with child rearing throughout history. By the same token, it does not take a genius to provide a child with a stimulating, nurturing environment. The average differences in environment across the cognitive classes are large and in many ways troubling, but, in percentage terms, they explain little of the variance. Abundant examples of excellent parents may be found through all but the very lowest range of cognitive ability.
The Role of Education
We conclude, as usual, by considering the role of education through the high school graduate and college graduate subsamples. Holding maternal age and the mother’s socioeconomic background constant at their means, college graduates tend to do well, no matter what their cognitive ability (within their restricted range), even though cognitive ability retains a statistically significant relationship. Within the high school sample, the effects of cognitive ability are plain; the odds of being in the bottom decile on the HOME index for the child of a mother of average socioeconomic background drop from 15 percent for a high school graduate at the 2d IQ centile to 5 percent for a comparable person at the 98th IQ centile. As in the earlier analyses, the most important impact of cognitive ability within the h
igh school graduates seems to be at the low end. Socioeconomic background also continues to play an important independent role, but less than IQ.
Some More Complications
The HOME inventory has two components—a Cognitive Stimulation score and an Emotional Support score—both adapted to three separate age groups (under 3, 3 to 5, and over 5 years of age). We conducted a variety of analyses to explore the subtests’ roles for different age groups. Briefly, the mother’s IQ had the dominant role in determining the Emotional Support score for children through the age of 5, whereas its role in determining Cognitive Stimulation was roughly coequal with education and socioeconomic background—the opposite of what one might predict. Maternal IQ was especially important for Emotional Support to the 3- to 5-year-old group. It would be worthwhile for investigators to explore with other data the NLSY’s indications that parental IQ is especially important for the home environment from ages 3 to 5, and the peculiar finding that parental IQ is more important for Emotional Support than for Cognitive Stimulation.
DEVELOPMENTAL OUTCOMES
The NLSY also administered batteries of tests regarding the developmental outcomes for the children of NLSY mothers. We review several indicators briefly, then present a summary index showing the interrelationships the mother’s cognitive ability, socioeconomic background, poverty, and welfare.
The Bell Curve: Intelligence and Class Structure in American Life Page 26