Psychohistory is the name of a fictional science in Isaac Asimov's Foundation Trilogy universe, which combined history, psychology and mathematical statistics to create a (nearly) exact science of the behavior of very large populations of people, such as the Galactic Empire. Asimov used the analogy of a gas - In a gas, the motion of a single molecule is very difficult to predict, but the mass action of the gas can be predicted to a high level of accuracy. Asimov applied this concept to the population of the fictional Galactic Empire, which numbered in the quadrillions. The character responsible for the science's creation, Hari Seldon, established two postulates: that the population whose behaviour was modeled should be sufficiently large and that they should remain in ignorance of the results of the application of psychohistorical analyses.
Later on in his career, Asimov described historical (pre-Seldon) origins of psychohistory. In The Robots of Dawn, which takes place thousands of years before Foundation, he describes roboticist Han Fastolfe's attempts to create the science based on careful observation of others, particularly his daughter Vasilia. In Prelude to Foundation we learn that it was in fact one of Fastolfe's robots, R. Daneel Olivaw, that manipulated Seldon into practical application of this science.
As a precursor to Asimov's psychohistory, one of Arthur Conan Doyle's Sherlock Holmes novels, a character describes the possibility of forecasting the behaviour of society using mathematical means.
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