In Some Thoughts Concerning Education, John Locke made an uncharacteristic but striking observation:

This study deals with a group of late nineteenth and twentieth century psychologists who spent their lives carrying out Locke's injunctions. They agreed that children differed in their innate capacities; tried to develop a scientific understanding of their ‘natures and aptitudes’; devised ‘Tryals’ intended to see ‘what turn they easily take’; and insisted that education should be based on each individual's ‘Natural Genius’.

The argument focuses on three main themes: the emergence of the profession of educational psychology; the history of ideas about children's mental development, in particular the development of the subnormal and the gifted; and the role of psychological experts in formulating educational policy. It tries to relate the history of psychology to the history of the meritocratic ideal – the idea that educational opportunity should be determined by natural ability rather than personal contacts – suggesting that both the rise and the decline of mental measurement were bound up with the political career of this concept.