Psychohistory is the name of a fictional science in Isaac Asimov's Foundation 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.
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.
Psychohistory is the mathematical study of the reactions of human conglomerates in response to economic and social stimuli. Psychohistory was developed by Hari Seldon and expanded through the efforts of Gaal Dornick and other mathematicians. Using psychohistory, Seldon predicted the fall of the Galactic Empire and developed Seldon's Plan to shorten the interregnum.
The character responsible for the science's creation, Hari Seldon, established three Theorems of Psychohistorical Quantitivity:
- The population under scrutiny is oblivious to the existence of the science of Psychohistory.
- The time periods dealt with are in the region of 3 generations.
- The population must be in the billions (±75 billions) for a statistical probability to have a psychohistorical validity.
- That Human Beings are the only intelligent life in the Universe.
In order for Seldon's mathematics to be valid, he had to make certain assumptions about the Galaxy and its inhabitants. Those assumptions, as stated by Ebling Mis, in 301FE, are:
- That there would be no fundamental change in human society over the thousand years of the Plan and
- That human reactions to stimuli would remain constant.