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These results show not only that siblings are different but thatthe home environment has had little or no net effect on the measuredoutcomes. If being reared by conscientious parents, for example,tended to make children more conscientious, then two children rearedby conscientious parents should, on average, both be moreconscientious than two reared by careless parents. Therefore, twochildren reared in the same home should be significantly more alike inconscientiousness than two reared in different homes, which is exactlywhat the studies do not find (Bouchard, 1994). The same results alsorule out the possibility that being reared by conscientious parentsmakes children conscientious on average. The bottom-lineeffect of the shared home environment on conscientiousness is notnoticeably different from zero. That is why knowledgeabledevelopmentalists (e.g., Collins, Maccoby, Steinberg, Hetherington,& Bornstein, 2000; Vandell, 2000) are now showing a diminishedinterest in looking for main effects and a keen interest ingene–environment interactions. If being reared by conscientiousparents makes children with one kind of temperament more conscientiousand those with another kind of temperament less conscientious, thenparents might have an influence after all, even in the absence of maineffects.

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