Hanlon's razor is an adage or rule of thumb that states "never attribute to malice that which is adequately explained by stupidity." Known in several other forms, it is a philosophical razor that suggests a way of eliminating unlikely explanations for human behavior. It is probably named after Robert J. Hanlon, who submitted the statement to Murphy's Law Book Two (1980). Similar statements have been recorded since at least the 18th century.
A similar quotation appears in Robert A. Heinlein's novella Logic of Empire (1941). The character "Doc" in Heinlein's story described the "devil theory" fallacy, explaining, "You have attributed conditions to villainy that simply result from stupidity."
The quotation as such was a submission credited in print to Robert J. Hanlon of Scranton, Pennsylvania, for a compilation of various jokes related to Murphy's law that were published in Arthur Bloch's Murphy's Law Book Two: More Reasons Why Things Go Wrong! (1980). It is unknown whether Hanlon knew of Heinlein's story or whether he independently constructed the phrase.
Hanlon's razor became well-known after its inclusion in the Jargon File, a glossary of computer programmer slang, since 1990. Later that same year, the Jargon File editors noted lack of knowledge about the term's derivation and the existence of a similar epigram by William James. In 1996, the Jargon File entry on Hanlon's Razor noted the existence of the phrase in Heinlein's novella, with speculation that Hanlon's Razor might be a corruption of "Heinlein's Razor". The link to Murphy's law was described in a pair of 2001 blog entries by Quentin Stafford-Fraser, citing emails from Joseph E. Bigler. Subsequently, in 2002, the Jargon File entry noted the same. Current Jargon File refers to it as a "Murphyism".
Other variations of the idea
Earlier attributions to the idea go back to at least the 18th century. Johann Wolfgang von Goethe wrote in the first entry of his influential epistolary novel The Sorrows of Young Werther (1774, first English translation 1779): "[...] Mißverständnisse und Trägheit vielleicht mehr Irrungen in der Welt machen als List und Bosheit. Wenigstens sind die beiden letzteren gewiß seltener." ('[...] misunderstandings and lethargy perhaps produce more wrong in the world than deceit and malice do. At least the latter two are certainly rarer.')  An alternate expression of the idea comes from Jane West, in her 1812 novel The Loyalists: An Historical Novel: "Let us not attribute to malice and cruelty what may be referred to less criminal motives.": 134 A similar quote is also misattributed to Napoleon. Andrew Roberts, in his biography of Winston Churchill, quotes from Churchill's correspondence with King George VI in February 1943 regarding disagreements with Charles De Gaulle: "'His 'insolence ... may be founded on stupidity rather than malice.'": 771
- Arthur Bloch (1980). Murphy's Law Book Two: More Reasons Why Things Go Wrong!. Price Stern Sloan. p. 52. ISBN 9780417064505.
- Eric S. Raymond, ed. (24 July 1996). "The Jargon File, Version 4.0.0". jargon-file.org. Retrieved 19 July 2017.
- Robert Heinlein (1 March 1941). "Logic of Empire". Astounding Science-Fiction. Vol. 27, no. 1. p. 39. Retrieved 8 August 2018.
- Guy L. Steele; Eric S. Raymond, eds. (12 June 1990). "The Jargon File, Version 2.1.1 (Draft)". jargon-file.org. Retrieved 19 July 2017.
- Eric S. Raymond; Guy L. Steele, eds. (15 December 1990). "The Jargon File, Version 2.2.1". jargon-file.org. Retrieved 19 July 2017.
- Stafford-Fraser, Quentin (26 November 2001). "[untitled]". Retrieved 19 July 2017.
- Stafford-Fraser, Quentin (4 December 2001). "The origins of Hanlon's Razor". Retrieved 19 July 2017.
- Eric S. Raymond, ed. (3 March 2002). "The Jargon File, Version 4.3.2". jargon-file.org. Retrieved 19 July 2017.
- "Hanlon's Razor". Jargon File. Eric S. Raymond. 3 March 2002. Retrieved 19 July 2017.
- Livraghi, Giancarlo (2004). Il potere della stupidità. Pescara, Italy: Monti & Ambrosini SRL. p. 1. ISBN 9788889479131.
- Selin, Shannon (14 July 2014). "Napoleon Misquoted - Ten Famous Things Bonaparte Never Actually Said". MilitaryHistoryNow.com. Retrieved 12 April 2019.
- Johann Wolfgang von Goethe (1774). Die Leiden des jungen Werthers or The Sufferings of Young Werther. Translated by Bayard Quincy Morgan. p. 14.
- West, Jane (1812). The Loyalists: An Historical Novel. Vol. 2. Boston. ISBN 9780665500428.
- Roberts, Andrew (2019). Churchill: Walking with Destiny. New York: Penguin Books. ISBN 9781101981009.