Research on individual differences in mental abilities is discussed from three viewpoints: the psychometric structure of ability differences, the predictive validity of mental test scores, and some putative causes of psychometric intelligence differences in terms of psychometric and cognitive components and biological indices. A hierarchical descriptive structure for mental ability differences, as it emerged in the 1980s and 1990s, prominently displays British discoveries and suggestions, especially those of Spearman, Burt and P. E. Vernon from the first half of the 20th century. Galton and Spearman's largely unproductive search for the origins of ability differences has seen new activity since the 1970s, and there are several replicable associations that are yet to be explained. Over the 20th century the emphasis has been on measuring mental ability differences; at its beginning there was an emphasis (largely British) on understanding psychometric intelligence. The new century is likely to see a continuation of this re-emphasis on explaining human ability differences.