The Psychological Bulletin

Data from 14 nations reveal IQ gains ranging from 5 to 25 points in a single generation. Some of the largest gains occur on culturally reduced tests and tests of fluid intelligence. The Norwegian data show that a nation can make significant gains on a culturally reduced test while suffering losses on other tests. The Dutch data prove the existence of unknown environmental factors so potent that they account for 15 of the 20 points gained. The hypothesis that best fits the results is that IQ tests do not measure intelligence but rather a correlate with a weak causal link to intelligence. This hypothesis can also explain differential trends on various mental tests, such as the combination of IQ gains and Scholastic Aptitude Test losses in the United States. Over the last decade, scholars in many countries—largely un-known to one another—have been measuring IQ trends from one generation to the next. In this article their results are col-lected and analyzed so as to reap the benefits comparative data usually yield. It is also argued that this mass of data poses fun-damental problems for developmental psychology, primarily concerning what factors have the most potent effect on IQ, what IQ tests measure, and how IQ tests should be used in making between-groups comparisons. Data Collection The method used to collect data can be simply put. Question-naires, letters, or personal appeals—usually a combination of all three—were sent to all those researchers known to be inter-ested in IQ trends on the basis of scholarly correspondence and the exchange of publications. One-hundred sixty-five scholars from 35 countries were contacted. They came from Europe—