Introduction

This book is concerned with a single major question: are scores on intelligence tests (I.Q.'s) heritable? The answer, in the consensus view of most intelligence testers, is that about eighty percent of individual variation in I.Q. scores is genetically determined. That is not a new conclusion. Pearson, writing in 1906, before the widespread use of the I.Q. test, observed that "the influence of environment is nowhere more than one-fifth that of heredity, and quite possibly not one-tenth of it."2 Herrnstein, reviewing the history of intelligence testing to 1971, concluded, "We may, therefore, say that 80 to 85 percent of the variation in I.Q. among whites is due to the genes."3