"The Bell Curve" declares that studies of separated identical twins--the "purest" of "direct" methods for estimating IQ heritability--indicate a value of +.75-+.80. But, the main study cited suggests a heritability of "two-thirds" for the middle class, and Herrnstein and Murray neglect to mention numerous…

The Ramey and Haskins intervention experiment is examined. Narrow transfer of training from cognitive intervention techniques to IQ test performance in early childhood, rather than enhancement of the g factor itself, is hypothesized as a cause of the typical fadeout of early IQ gains in later childhood. (Author/RD)

Extensive data on twins, nontwin siblings, siblings separated by adoption, and parent-child similarity indicate that the heritability of intelligence is closer to .50 than to .70. Differences could be due to environmental or genetic changes in the population, or to methodology. (Author/CP)