“At least two thirds of our miseries spring from human stupidity, human malice, and those great motivators and justifiers of malice and stupidity, idealism, dogmatism and proselytizing zeal on behalf of religious or political idols.”

Murphy's Law Book Two : More Reasons Why Things Go Wrong! (1980) ISBN 0843106743 by Arthur Bloch page 52.
This statement has often been referred to as Hanlon's Razor, though the author himself has remained relatively unknown, and some theories arose that "Hanlon" might have been a corruption of "Heinlein". Hanlon actually was a winner in a contest to come up with further statements similar to "Murphy's Law", for publication in this particular book. This phrase or very similar statements have also been attributed to William James, Napoleon Bonaparte, Richard Feynman (who might well have quoted it) and others.
Similar statements have been made by Goethe, and indeed, Robert Heinlein:
In The Sorrows of Young Werther Goethe declared, "Misunderstandings and neglect occasion more mischief in the world than even malice and wickedness. At all events, the two latter are of less frequent occurrence."
In his story Logic of Empire (1941) Heinlein declares: "You have attributed conditions to villainy that simply result from stupidity". He calls this the "devil theory" of sociology. His character Lazarus Long also voices a variation on the theme in the novel Time Enough for Love: "Never underestimate the power of human stupidity."
Variants of the phrase which have been variously attributed to this wide assortment of authors include:
Never ascribe to malice that which can be adequately explained by stupidity.
Never ascribe to malice that which can be explained by incompetence.
Never ascribe to malice that which is adequately explained by stupidity.

The Bonhoeffer Reader https://www.google.com/books/edition/The_Bonhoeffer_Reader/CNZgAwAAQBAJ?hl=en&gbpv=1&pg=PA766, p. 766
Letters and Papers from Prison (1967; 1997), On Stupidity

Source: Rule 34 (2011), Chapter 31, “Dominoes Fall” (p. 322)

Source: The Demon-Haunted World: Science as a Candle in the Dark

As discussed in this entry from The Quote Investigator http://quoteinvestigator.com/2010/05/04/universe-einstein/#more-173, the earliest published attribution of a similar quote to Einstein seems to have been in Gestalt therapist Frederick S. Perls' 1969 book Gestalt Theory Verbatim, where he wrote on p. 33: "As Albert Einstein once said to me: 'Two things are infinite: the universe and human stupidity.' But what is much more widespread than the actual stupidity is the playing stupid, turning off your ear, not listening, not seeing." Perls also offered another variant in his 1972 book In and Out the Garbage Pail, where he mentioned a meeting with Einstein and on p. 52 http://books.google.com/books?id=HuxFAAAAYAAJ&q=human+stupidity#search_anchor quoted him saying: "Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." However, Perls had given yet another variant of this quote in an earlier book, Ego, Hunger, and Aggression: a Revision of Freud’s Theory and Method (originally published 1942, although the Quote Investigator only checked that the quote appeared in the 1947 edition), where he attributed it not to Einstein but to a "great astronomer", writing: "As modern times promote hasty eating to a large extent, it is not surprising to learn that a great astronomer said: 'Two things are infinite, as far as we know – the universe and human stupidity.' To-day we know that this statement is not quite correct. Einstein has proved that the universe is limited." So, the later attributions in 1969 and 1972 may have been a case of faulty memory, or of intentionally trying to increase the authority of the quote by attributing it to Einstein. The quote itself may be a variant of a similar quote attributed even earlier to the philosopher Ernest Renan, found for example in The Public: Volume 18 from 1915, which says on p. 1126 http://books.google.com/books?id=cTPmAAAAMAAJ&pg=PA1126#v=onepage&q&f=false: "He quotes the saying of Renan: it isn't the stars that give him an idea of infinity; it is man's stupidity." (Other examples of similar attributions to Renan can be found on this Google Books search http://www.google.com/search?q=renan+infinity+stupidity&btnG=Search+Books&tbm=bks&tbo=1.) Renan was French so this is presumably intended as a translation, but different sources give different versions of the supposed original French quote, such as "La bêtise humaine est la seule chose qui donne une idée de l'infini" (found for example in Réflexions sur la vie, 1895-1898 by Remy de Gourmont from 1903, p. 103 http://books.google.com/books?id=RtrtAAAAMAAJ&pg=PA103#v=onepage&q&f=false, along with several other early sources as seen in this search http://www.google.com/search?q=%22humaine+est+la+seule+chose+qui%22+renan&btnG=Search+Books&tbm=bks&tbo=1) and "Ce n'est pas l'immensité de la voûte étoilée qui peut donner le plus complétement l'idée de l'infini, mais bien la bêtise humaine!" (found in Broad views, Volume 2 from 1904, p. 465 http://books.google.com/books?id=9NEaAQAAMAAJ&pg=PA465#v=onepage&q&f=false). Since these variants have not been found in Renan's own writings, they may represent false attributions as well. They may also be variants of an even older saying; for example, the 1880 book Des vers by Guy de Maupassant includes on p. 9 http://books.google.com/books?id=cQUvAAAAMAAJ&pg=PP21#v=onepage&q&f=false a quote from a letter (dated February 19, 1880) by Gustave Flaubert where Flaubert writes "Cependant, qui sait? La terre a des limites, mais la bêtise humaine est infinie!" which translates to "But who knows? The earth has its boundaries, but human stupidity is infinite!" Similarly the 1887 book Melanges by Jules-Paul Tardivel includes on p. 273 http://books.google.com/books?id=n9cOAAAAYAAJ&pg=PA273#v=onepage&q&f=false a piece said to have been written in 1880 in which he writes "Aujourd'hui je sais qu'il n'y a pas de limites à la bêtise humaine, qu'elle est infinie" which translates to "today I know that there is no limit to human stupidity, it is infinite."
Disputed
Variant: "Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." Earliest version located is in Technocracy digest: Issues 287–314 from 1988, p. 76 http://books.google.com/books?id=L7LnAAAAMAAJ&q=%22sure+about+the+former%22#search_anchor. Translated to German as: "Zwei Dinge sind unendlich: das Universum und die menschliche Dummheit. Aber beim Universum bin ich mir nicht ganz sicher." (Earliest version located is Arndt-Michael Meyer, Die Macht der Kürze, Books on Demand GmbH, 2004, p. 14 http://books.google.gr/books?id=12DW-RBKTW8C&pg=PA14&dq=%22Zwei+Dinge+sind+unendlich:+das+Universum+und+die+menschliche+%22+arnd&hl=en&sa=X&ei=gquJUsrYBomM7AapmYGgCQ&ved=0CC8Q6wEwAA#v=onepage&q=%22Zwei%20Dinge%20sind%20unendlich%3A%20das%20Universum%20und%20die%20menschliche%20%22%20arnd&f=false.)
Variant: Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.

Sensibility.

1930s, Address at San Diego Exposition (1935)
Context: Several centuries ago the greatest writer in history described the two most menacing clouds that hang over human government and human society as "malice domestic and fierce foreign war." We are not rid of these dangers but we can summon our intelligence to meet them. Never was there more genuine reason for Americans to face down these two causes of fear. "Malice domestic" from time to time will come to you in the shape of those who would raise false issues, pervert facts, preach the gospel of hate, and minimize the importance of public action to secure human rights or spiritual ideals. There are those today who would sow these seeds, but your answer to them is in the possession of the plain facts of our present condition.

Source: Lyonesse Trilogy (1983-1989), The Green Pearl (1985), Chapter 6, section 5 (p. 451)

Manchester Guardian, 2 July 1934, quoted in Bernard Donoughue and George Jones, "Herbert Morrison: Portrait of a Politician" (Phoenix Press, 2001), p. 184.