The G Factor: The Science of Mental Ability

By Arthur R. Jensen | Go to book overview

evidence. Rarely will any single item of evidence prove pivotal in determining whether a prosecutor's scenario or the defense's alternative is most likely to be correct. Many single details may actually fail to favor one scenario over another. The most probable account, instead, is the one which is the most internally consistent--the one in which all the facts mesh together most neatly with one another and with the motives in the case. Of paramount importance is the economy of explanation. There are always alternative explanations of any single isolated fact. The greater the number of special explanations required in a narrative, however, the less probable its accuracy. An effective scenario almost always has a compelling facility to explain a chain of facts with a minimum of such special explanations. Instead the pieces of the puzzle should fall into place.[ 112, p. 2]

Rushton, 1989.
Jensen, 1994f.
The rate of change in the frequency of a particular allele in a population depends partly on its relative fitness, or reproductive advantage. It also depends on the amount of variation at that gene locus in the population; a rare allele, regardless of its relative fitness, spreads slowly at first and then at an accelerating rate. To take an extreme example, say that a rare allele occurs with a frequency of only 1 percent in a given population, and this allele enhances fitness by only 1 percent (i.e., those who possess the allele have 1 percent more progeny than those who do not possess it). Then, over the course of 1,000 generations, assuming the same reproductive advantage is maintained over this period, the percentage of individuals in the population who possess the allele will have risen from the original 1 percent up to 99 percent. If a change in the physical or social environment for some reason made the same allele disadvantageous in terms of fitness, its frequency in the population would gradually decrease, either to zero or to its low frequency of occurrence through spontaneous mutation of another allele at the same chromosomal locus.
One often hears it said that the genetic differences within racial groups (defined as statistically different breeding populations) is much greater than the differences between racial groups. This is true, however, only if one is comparing the range of individual differences on a given characteristic (or on a number of characteristics) within each population with the range of the differences that exist between the means of each of the separate populations on the given characteristic. In fact, if the differences between the means of the various populations were not larger than the mean difference between individuals within each population, it would be impossible to distinguish different populations statistically. Thinking statistically in terms of the analysis of variance, if we obtained a very large random sample of the world's population and computed the total variance (i.e., the total sum of squares based on individuals) of a given genetic character, we would find that about 85 percent of the total genetic variance exists within the several major racial populations and 15 percent exists between these populations. But when we then divide the sum of squares (SS) between populations by its degrees of freedom to obtain the mean square (MS) and we do the same for the sum of squares within populations, the ratio of the two mean squares, i.e., Between MS/Within MS, (known as the variance ratio, or F ratio, named for its inventor, R. A. Fisher) would be an extremely


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The G Factor: The Science of Mental Ability
Table of contents

Table of contents

  • Title Page iii
  • Contents vii
  • Preface ix
  • Acknowledgments xiii
  • Chapter 2 - The Discovery of G 18
  • Notes 39
  • Chapter 3 - The Trouble with "Intelligence" 45
  • Notes 68
  • Chapter 4 - Models and Characteristics of G 95
  • Chapter 5 - Challenges to G 105
  • Notes 133
  • Chapter 6 - Biological Correlates of G 137
  • Notes 165
  • Chapter 7 - The Heritability of G 169
  • Notes 197
  • Chapter 8 - Information Processing and G 203
  • Notes 261
  • Chapter 9 - The Practical Validity of G 270
  • Notes 301
  • Chapter 10 - Construct, Vehicles, and Measurements 306
  • Notes 344
  • Chapter 11 - Population Differences in G 350
  • Notes 402
  • Chapter 12 - Population Differences in G: Causal Hypotheses 418
  • Notes 516
  • Chapter 13 - Sex Differences in G 531
  • Notes 542
  • Chapter 14 - The G Nexus 544
  • Notes 579
  • Appendix A - Spearman's "Law of Diminishing Returns" 585
  • Appendix B - Method of Correlated Vectors 589
  • Appendix C - Multivariate Analyses of a Nexus 593
  • References 597
  • Name Index 635
  • Subject Index 643
  • About the Author 649


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