Nurturing doubts

February 26, 1999

In Michael Rutter's review (THES, January 22), my book was called The Nature Assumption. The actual title is The Nurture Assumption. It is a small difference but an important one, because Professor Rutter and I do not disagree about the importance of "nature". Our disagreement has to do with what I call "environment" but most people nowadays call "nurture".

The nurture assumption is the firmly held belief that parents have the power to shape or modify their children's personality and intelligence. In my book, I showed how this unquestioned assumption leads researchers astray by predisposing them to interpret ambiguous evidence as proof of their a priori beliefs. Even researchers as sophisticated as Rutter fall into this trap.

For example, he said: "A correlation does not provide a good measure of the strength of an effect because it is so dependent on the number of individuals in the study to whom the risk factor applies." In truth, a correlation cannot measure the strength of an effect. Correlations do not provide unambiguousevidence of causes or effects.

Almost all the data Professor Rutter cited in his review are ambiguous. Most are correlational; most were collected with research methods that cannot distinguish among alternative explanations of the correlation.

Science progresses by testing hypotheses, but parental influence is not regarded as a hypothesis: it is treated as a "given". The proper way to proceed would be to regard the hypothesis of zero parental influence not as an extreme or provocative position but as the null hypothesis to be tested and rejected. It has not been done. After 50 years of assiduous efforts, researchers have still not come up with solid evidence that would enable them to reject, with any degree of confidence, the null hypothesis that parents have no lasting effects on their children. It is time to look elsewhere for the formative influences that shape personality and intelligence Judith Rich Harris Middletown, New Jersey United States

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