Abstract

Variation in cognitive abilities has had an unusual status ever since the birth of psychology. For a number of reasons, it developed separately from mainstream experimental psychology, leading to a duality that Cronbach called the ‘two disciplines of scientific psychology’. The applied measurement of abilities became a huge and successful industry which resulted in the development of advanced methods of statistical analysis and modelling. This came at the cost of appropriate theoretical foundations. The concept of working memory capacity changed the scene: measures of individual differences in an established cognitive construct were able to predict important real-life outcomes. In turn, test constructors became more and more influenced by theory. The paper surveys the history of this separation and the current efforts that aim to end it, emphasizing the relevance of William Stern’s century old, but mostly ignored proposal for a common framework.