The Science and Politics of I.Q.

By Leon J. Kamin | Go to book overview

3
SEPARATED IDENTICAL TWINS

Political wisdom can be garnered from a study of twins.

- ProfessorN. D. M. Hirsch, 19301

Probably the best known evidence supporting the idea that I.Q. scores are inherited comes from observations made on separated identical twins. That evidence seems especially powerful, because it is based on fewer--and simpler-- arbitrary assumptions than must be made to interpret other forms of data. There are two very different types of twins. When a single sperm fertilizes a single ovum, it sometimes happens that two different individuals develop. These individuals are identical, or monozygotic (MZ), twins. They presumably have identical genes. They are necessarily of the same sex, and their physical similarities are typically very striking. The other type of twin pair occurs when two different sperms fertilize two different ova at about the same time. The mother will then bear two different individuals, known as fraternal, or dizygotic (DZ), twins. Though born at the same time, these two individuals will be no more genetically alike than are ordinary brothers and sisters. They will have in common, on average, only half their genes. They may be same-sexed or opposite-sexed, and may be very dissimilar physically.

The MZ twin pairs seem uniquely equipped to serve as subjects in experiments concerned with heredity. They are the only paired individuals who exist with identical genes. That means that any differences which exist between a pair of MZ twins must be exclusively of nongenetic origin. To experiment upon heredity, it would be logical, though immoral, deliberately to subject members of an MZ twin

-33-

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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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