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Despite Kaufman, Raven’s Progressive Matrices and the Wechsler subtest Similarities are tests whose gains call for special explanation. The spread of “scientific spectacles” is the key, but its explanatory potential has been exhausted. Three trends force us to look elsewhere: (a) gains on Wechsler subtests such as Picture Arrangement, (b) gains in developed nations persisting into the 21st century, (c) the growing gap between the active vocabularies of parents and their children.
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James R. Flynn (2010) studied this question.
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