PSYCHOHISTORY -- Gaal Dornick, using nonmathematical concepts, has defined Psychohistory to be that branch of mathematics which deals with the reactions of human conglomerates to fixed social and economic stimuli. Implicit in all these definitions the assumption that the human conglomerate being dealt with s sufficiently large for valid statistical treatment. The necessary size of such a conglomerate may be determined by Seldon's First Theorem which ... a further necessary assumption is that human conglomerate be itself unaware of Psychohistory analysis in order that its reactions be truly random. The basis of all valid Psychohistory lies in the development of the Seldon functions which exhibit properties congruent to those of such social and economic forces as ...

-Encyclopedia Galactica

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.

Golan Trevize also discovered a fourth, unstated, axiom in 499FE (Foundation and Earth),

  • 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.