The authors consider Professor Jensen's hypothesis that inherited factors may be"implicated" in observed racial differences in measured intelligence. They argue that even if one chooses to accept Professor Jensen's estimates of the proportion of variance in intelligence accounted for by heredity, environment, and their interaction,his hypothesis is not substantiated by his own data. They go on to say that the parameter estimates are highly suspect, given the small sample size of the twin studies and the way disparate studies were combined. The authors simulated, on a computer, the process of studying twins and found that the statistical procedures employed in these studies of intelligence yield quite unstable estimates. In particular,the estimate of the interaction effect is quite unreliable, both because of sample size, and because Jensen chose a statistical model which would attribute some interaction to the main variables—heredity and environment. Finally, the authors propose that the studies of intelligence reported by Professor Jensen ignore the reality of feedback loops, initiated by physical differences, and enhanced by processes of social differentiation in our society.

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