Madame de Tencin said that intelligent people often erred in their conduct because they could never believe that the world in general is as stupid as it is.

[Mme de Tencin disait que les gens d’esprit faisaient beaucoup de fautes en conduite, parce qu’ils ne croyaient jamais le monde assez bête, aussi bête qu’il l’est.]

Nicolas Chamfort
Nicolas Chamfort (1741-1794) French writer, epigrammist (b. Nicolas-Sébastien Roch)
Products of Perfected Civilization [Produits de la Civilisation Perfectionnée], Part 2 “Characters and Anecdotes [Caractères et Anecdotes],” ¶ 715 (1795) [tr. Merwin (1969)]
    (Source)

(Source (French)). Alternate translations:

Mme de Tencin said that people with spirit make many mistakes in their actions because they never believe that society is brutish enough, as brutish as it is.
[tr. Siniscalchi (1994)]

Madame de Tencin said that many very clever people made social blunders because they could never believe that society was quite as stupid as it really was.
[tr. Parmée (2003), ¶382]

The great mistake made by intelligent people is to refuse to believe that the world is as stupid as it is.
[Forbes]