[Correction Notice: An erratum for this article was reported in Vol 101(3) of Psychological Bulletin (see record 2008-10701-001). In this article, it was wrongly stated that that U. Schallberger put forward a hypothesis or hypotheses concerning the magnitude of Swiss IQ gains; in fact, he did not. The erratum includes the author's clarification.] 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 proved 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 difficult trends on various mental tests, such as the combination of IQ gains and Scholastic Aptitude Test losses in the United States. (PsycINFO Database Record (c) 2016 APA, all rights reserved)