Bell Curve

2 P 6 Cognitive Classes and Social Behavior Parenting 2 1 7A low IQ is a major risk factor, whereas the mother's socioeconomicbackground is irrelevant. A mother at the 2d centile of IQ had a 7 percentchance of giving birth to a lowdbirth-weight baby, while a motherat the 98th percentile had less than a 2 percent chance.Adding Poverty. Poverty is an obvious potential factor when trying toexplain low birth weight. Overall, poor white mothers (poor in the yearbefore birth) had 61 low-birth-weight babies per 1,000, while otherwhite mothers had 36. But poverty's independent role was small and statisticallyinsignificant, once the other standard variables were taken intoaccount. Meanwhile, the independent role of IQ remained as large, andthat of socioeconomic background as small, even after the effects ofpoverty were extracted.Can Mothers Be Too Smart for Their Own Good?The case of low birth weight is the first example of others you will see inwhich the children of white women in Class I have anomalously had scores.The obvious, but perhaps too obvious, culprit is sample size. The percentageof low-birth-weight babies for Class 1 mothers, calculated using sampleweights, was produced by just two low-birth-weight hahies out ofseventy-four birth^."^' The sample sizes for white Class i mothers in theother analyses that produce anomalous results are also small, sometimesunder fifty and always under one hundred, while the sample si:es for themiddle cognitive classes number several hundred or sometimes thousands.On the other hand, perhaps the children of mothers at the very top ofthe cognitive distribution do In fact have different tendencies than the restof the range. The poss~bility is sufficiently intriguing that we report theanomalous data despite the small sample sizes, and hope that others willexplore where we cannot. In the logistic regression analyses, where eachcase is treated as an individual unit (not grouped into cognitive classes),these problems of sample size do not arise.Adding mother's age at the time of birth. It is often thought that veryyoung mothers are vulnerable to having low-birth.weight babies, nomatter how good the prenatal care may be.55 This was not true in theNLSY data for white women, however, where the mothers of low-birthweight babies and other mothers had the same mean (24.2 years).In sum, neither the mother's age in the NLSY cohort, nor age at birthof the child, nor poverty status, nor socioeconomic background had anyappreciable relationship to her chances of giving birth to a low-birthweightbaby after her cognitive ability had been taken into account.Adding education. Among high school graduates (no more, no less)in the NLSY, a plot of the results of the standard analysis looks visuallyidentical to the one presented for the entire sample, but the sample oflow-birth-weight babies was so small that the results do not reach statisticalsignificance. Among the college graduates, low-birth-weight babieswere so rare (only six out of 277 births to the white college sample)that a multivariate analysis produced no interpretable results. We donot know whether it is the education itself, or the self-selection thatgoes into having more education, that is responsible for their low incidenceof underweight babies.Infant MortalityThough we have not been able to find any studies of cognitive abilityand 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 exerciseboth watchfulness and judgment. It takes more than love to childproofa house effectively; it also takes knowledge and foresight. It takes intelligenceto decide that an apparently ordinary bout of diarrhea has goneon long enough to make dehydration a danger; and so on. Nor is simpleknowledge enough. As pediatricians can attest, it may not be enoughto tell new parents that infants often spike a high fever, that suchepisodes do not necessarily require a trip to the hospital, hut that theyrequire careful attention lest such a routine fever become life threatening.Good parental judgment remains vital. For that matter, the problemfacing pediatricians dealing with children of less competent parentsis even more basic than getting them to apply good judgment: It is toget such mothers to administer the medication that the doctor has provided.This rationale is consistent with the link that has been found betweeneducation and infant mortality. In a study of all births registeredin California in 1978, for example, infant deaths per 1,000 to whitewomen numbered 12.2 for women with less than twelve years of edu-