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The present study investigates the impact of changing fertility patterns on the Flynn effect. Intelligence test data comprised scores of army conscripts on an arithmetic, language and a Raven-similar test, and a composite score (General Ability). Family data of the conscripts enabled a decomposition of the population mean into effects of sibship size on the mean intelligence and the proportion of the persons comprising the various sibship sizes within each of 13 birth cohort groups (each comprising 3 birth years). Both the means within each sibship and the proportions of the different sibship sizes varied across cohorts. Estimated changes in means due to changing proportions of sibship sizes alone were calculated by fixing the mean intelligence test score within the different sibship sizes at the level of the oldest birth cohort (1938–1940) and letting the proportions of the different sibship sizes take their empirical values in each of the subsequent 12 three-year cohort groups. It is concluded that changing proportions of sibship sizes had a moderate effect both on General Ability and the subtest scores, and that most of the changes were connected to changing sibship means across cohorts.