Psychohistory, a fictional science in Isaac Asimov's Foundation universe, combines history, sociology, and mathematical statistics to make (nearly) exact predictions of the collective actions of very large groups of people, such as the Galactic Empire.
Psychohistory depends on the idea that, while one cannot foresee the actions of a particular individual, the laws of statistics as applied to large groups of people could predict the general flow of future events. Asimov used the analogy of a gas: an observer has great difficulty in predicting the motion of a single molecule in a gas, but can predict the mass action of the gas to a high level of accuracy. Physicists know this as the Kinetic theory. Asimov applied this concept to the population of his fictional Galactic Empire, which numbered a quintillion. The character responsible for the science's creation, Hari Seldon, established two axioms:
There is a third underlying axiom of Psychohistory, which is trivial and thus not stated by Seldon in his Plan:
The Prime Radiant projects the equations onto walls in some unexplained manner, but it does not cast shadows, thus allowing workers easy interaction. Control operates through the power of the mind, allowing the user to zoom in to details of the equations, and to change them. One can make annotations, but by convention all amendments remain anonymous.
To make a change, a student destined for speakerhood has to present an amendment to the plan. Five different boards would then check the mathematics rigorously. Students have to defend their proposals against concerted and merciless attacks. After two years the change will get reviewed again. If after the second examination it still passes muster the contribution becomes part of the Seldon Plan.
The Radiant, as well as being interactive, employs a type of colour-coding to equations within itself for ready comprehension by Psychohistorians.
Other colours have been imagined by fans, and mentioned by Asimov, such as:
In his later career, Asimov described some historical (pre-Seldon) origins of psychohistory. In The Robots of Dawn (1983), which takes place thousands of years before Foundation (1951), he describes roboticist Han Fastolfe's attempts to create the science based on careful observation of others, particularly of his daughter Vasilia. Prelude to Foundation (1988) suggests that one of Fastolfe's robots, R. Daneel Olivaw, manipulated Seldon into practical application of this science.
The fact that Seldon established a Second Foundation of mental-science adepts to oversee his Seldon Plan might suggest that even Seldon himself had doubts about the ultimate ability of a purely mathematical approach to predicting historical processes, and that he recognized that the development of psychic skills such as those used by the Mule, had the ability to invalidate the assumptions underlying his models. The Seldon methodology might therefore only work at a certain level of species-development, and would over time become less useful.
Psychohistory has one basic, underlying limitation which Asimov postulated for the first time on literally the last page of the final book in the Foundation series: psychohistory only functions in a galaxy populated only by humans. In Asimov's Foundation series, humans form the only sentient race that developed in the entire Milky Way Galaxy. Seldon developed psychohistory to predict the actions of large groups of humans. Even robots technically fall under the umbrella of psychohistory, because humans built them, and they thus represent more or less a human "action", or at least, possess a thought-framework similar enough to that of their human creators that psychohistory can predict their actions. However, psychohistory cannot predict the actions of a sentient alien race; their psychology may differ so much from that of humans that normal psychohistory cannot understand or predict their actions.
The end of the series offered two possibilities:
Asimov's ideas figure prominently in Donald Kingsbury's novel Psychohistorical Crisis, a re-imagining of the world of Isaac Asimov's Foundation trilogy, set after the establishment of the Second Empire.
In Fantastic Four #542, Mister Fantastic, involved in the Marvel Universe's Civil War event, reveals his real reason for supporting the superhero registration act which prompted the Civil War — his development of a working version of Isaac Asimov's fictional psychohistory-concept. Mister Fantastic's application of this science indicates to him that billions will die in escalating conflicts unless the act becomes law.
In Ghost Rider 2099 #1, a group of AIs predict that human society (and therefore the global network the AIs exist in) will crash in 2113. One of them mentions that Asimov conceived the idea of such a mathematical model.
Some individuals and groups, inspired by Asimov's psychohistory, seriously explore the possibility of a working psychohistory not unlike the one imagined by Asimov — a statistical study of history that could help in the formulation of some "theory of history" and perhaps become a tool of historical prediction. Such groups include "The psychohistory project" In 2002 a book appeared in Greek on the subject: "Psychohistory" (A tool for Historical Prediction) by Christos Z. Konstas, ISBN : 960-7928-72-5.
Psychohistory appears in the Traveller science-fiction role-playing game, released in 1977. The alien race known as the Hivers use extensive manipulation of other cultures based on psychohistorical data to achieve their own ends. Rumours ascribe the assassination of the Third Imperium's Emperor Strephon to a Hiver manipulation based on psychohistorical data indicating the eventual fall of the Third Imperium. Humans in the setting have also attempted to use psychohistory, but with less skill or success; the Psionic Suppressions (which turned public opinion within the human Imperium against those with paranormal mental abilities, forcing them to go into hiding) resulted, unknown to most, from an experiment in psychohistory that got out of control and went much farther than the experimenters intended.
Some literary critics have described Asimov's psychohistory as a reformulation, either for better or worse, of Karl Marx's theory of history (historical materialism) or of Kant's theory of controllable history, though Asimov denied any direct influence. Psychohistory also has echoes of modernization theory and of work in the social sciences that by the 1960s would lead to attempts at large-scale social prediction and control such as Project Camelot.