The Science and Politics of I.Q.

By Leon J. Kamin | Go to book overview

to reject the hypothesis of zero I.Q. heritability. The available data are at many critical points almost wholly unreliable. The correlation between separated siblings might be of particular interest since (under the false assumption of uncorrelated environments) its divergence from genetic expectation could be used to estimate heritability. The widely circulated assertion that the "median correlation" found in studies of separated sibs is .47 is, however, false. The two studies other than Burt's found much lower correlations, figures wholly consistent with zero heritability. The comparison of DZ and sib correlations is also theoretically relevant, but we have no very good estimate of the "true" I.Q. correlation for DZs. The published "medians" involve systematic biases that minimize this difference. When comparisons are made within the same study, the DZ correlation is perceptibly higher than that for sibs, with the exception of Burt's study. There is suggestive evidence that the singleton sibs of twins may resemble them much less than, in a genetic view, they should. The data for more remote kinship categories are very sparse and largely attributable to Burt's "assessments." There is thus no firm data base against which to assess genetic models that purport to predict the full range of kinship correlations. There is substantial evidence to suggest that the correlations for sibs and for parent-child are somewhere in the neighborhood of .50, but even at these points there is considerable variation from study to study. The genetic models thus have no basis on which to conclude whether or not dominance is present and cannot distinguish "dominance" from systematic environmental effects. The sib and parent-child correlations are in any event as consistent with an environmental as a genetic interpretation. The facts that twin correlations are high, and that MZs resemble one another more than DZs, are wholly consistent with the expectations of an environmental view. The data strongly suggest that male DZs display more within-pair I.Q. variance than do female DZs, and male DZs also report more dissimilar experiences than do female DZs. These facts undercut the theoretical basis for estimating I.Q. heritability from MZ and DZ correlations. The heritability estimates calculated from such data fluctuate wildly from study to study, and they include a theoretically absurd negative estimate, as well as theoretically absurd estimates larger than 100 percent. The orderliness attributed to median kinship correlations, and the cross- validating consistency said to characterize different methods of estimating heritability, are in part the product of systematic bias and in part wholly imaginary.


NOTES TO CHAPTER FOUR
1.
The Family-History Book, ed. C. B. Davenport, Eugenics Record Office Bulletin No. 7 (Cold Springs Harbor, N.Y., 1912), p. 62
2.
L. Erlenmeyer-Kimling and L. F. Jarvik, "Genetics and Intelligence: A Review," Science, 142, ( 1963), 1478
3.
Intelligence: Genetic and Environmental Influences, ed. R. Cancro, ( New York, Grune and Stratton, 1971), p. 185

-105-

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The Science and Politics of I.Q.
Table of contents

Table of contents

  • Title Page iii
  • Contents v
  • Acknowledgments vii
  • Introduction 1
  • NOTES TO INTRODUCTION 4
  • 1 - The Pioneers of I.Q. Testing in America 5
  • NOTES TO CHAPTER ONE 12
  • 2 - Psychology and the Immigrant 15
  • NOTES TO CHAPTER TWO 30
  • 3 - Separated Identical Twins 33
  • 4 - Kinship Correlations 73
  • NOTES TO CHAPTER FOUR 105
  • 5 - Studies of Adopted Children 111
  • NOTES TO CHAPTER FIVE 133
  • 6 - The Accuracy of Secondary Sources 135
  • NOTES TO CHAPTER SIX 157
  • 7 - I.Q. in the Uterus 161
  • NOTES TO CHAPTER SEVEN 173
  • Conclusion 175
  • NOTES TO CONCLUSION 179
  • Author Index 181
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