Psychohistory is a fictional science in Isaac Asimov's Foundation universe which combines history, sociology, and mathematical statistics to make general predictions about the future behavior of very large groups of people, such as a Galactic Empire. It was first introduced in the five short stories (1942–1944) which would later be collected as the 1951 novel Foundation?.

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:

  • that the population whose behaviour was modeled should be sufficiently large
  • that the population should remain in ignorance of the results of the application of psychohistorical analyses

There is a third underlying axiom of Psychohistory, which is trivial and thus not stated by Seldon in his Plan:

  • that Human Beings are the only sentient intelligence in the Galaxy.

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, though he did not (and could not) predict the appearance of the Mule himself. 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:

  • sentient races actually very rarely develop, such that only humans evolved in the Milky Way Galaxy, and in most other galaxies, it appears probable (given this assumption) that only one sentient race would develop. However, statistically two or more alien races might evolve in the same galaxy, leading them into inevitable conflict. The fighting in this other galaxy would only end when one race emerged the victor, and after the prolonged conflict with other races, would have developed an aggressive and expansionist mindset. In contrast, humans had never encountered another sentient species in the Milky Way Galaxy, so they never felt greatly compelled to expand to other galaxies, but instead to fight other humans over control of the Milky Way. Eventually, such an aggressive alien race would expand from galaxy to galaxy, and eventually try to invade the Milky Way Galaxy.
  • through genetic engineering, subsets of humanity could alter themselves so significantly from baseline humans that they could for all intents and purposes be considered "aliens". Specifically exemplifying this theory we find Asimov's Solarians: humans evolved from an old Spacer world who had genetically modified themselves into hermaphrodites with telekinetic mental powers.

Psychohistory in other Space Opera

House of Suns?: The 2008 novel House of Suns by Alastair Reynolds? features a device called the "Universal Actuary", which aims to predict the future of civilisations in a manner very similar to psychohistory. As the limits of slower than light travel prevent any interstellar civilisations from lasting very long, one of its most important uses is to determine how much longer a given civilisation will last.

Hyperion?: The Technocore, the AI civilisation in Dan Simmons's? 1989 novel Hyperion, is capable of statistically predicting future events to a very high degree of accuracy.

Star Trek: Deep Space Nine?: In the 1997 Star Trek: Deep Space Nine episode Statistical Probabilities, a think tank uses mathematics to predict the future in a manner likely to be a reference to Asimov.

Star Trek Preserver?: In the 2000 novel Preserver (Star Trek) by William Shatner? the science of psychohistory is used, and mentioned by name, by scholars at outpost Memory Alpha. Memory Alpha was shown in the Star Trek: Original Series episode The Lights of Zetar, although psychohistory was never mentioned in the episode.

Transformers?: Timelines: In the 'Shattered Glass' universe of 'Transformers: Timelines' created in 2005, Megatron uses math to predict the future in what is likely a reference to Asimov.

In 1995, Donald Kingsbury wrote "Historical Crisis", which he later expanded into a novel, Psychohistorical Crisis, published 2001. It takes place about 2,000 years after Foundation, after the founding of the Second Galactic Empire. It is set in the same fictional universe as the Foundation series, in considerable detail, but with virtually all Foundation-specific names either changed (e.g., Kalgan becomes Lakgan), or avoided (psychohistory is created by an unnamed, but often-referenced Founder). The novel explores the ideas of psychohistory in a number of new directions, inspired by more recent developments in mathematics and computer science, as well as by new ideas in science fiction itself.