Forbidden knowledge

Forbidden knowledge is information, sometimes in the form of forbidden books, to which access is restricted or deprecated for political or religious reasons. It differs from secret knowledge in that forbidden knowledge is commonly not secret, rather a society or various institutions will use repressive mechanisms to either completely prevent the publication of information they find objectionable or dangerous (censorship), or failing that, to try to reduce the public's trust in such information (propaganda). Public repression can create paradoxical situations where the proscribed information is generally common knowledge but publicly citing it is disallowed.

Background

A rich set of examples exist through history.

The Roman Catholic church forbids publication of books to which it has not granted Imprimatur.
Throughout the years of isolation in Japan and China all Western literature was forbidden.
Certain 20th century governments (e.g. communist nations in Eastern Europe and China) placed strong restrictions on foreign publications.
In the United States, conservative groups including Jerry Falwell's Moral Majority made several attempts to censor pro civil-rights and feminist works such as Our Bodies, Ourselves.

In many cases this resulted in people defending themselves by creating political jokes. Jokes throughout history have been a powerful instrument to undermine state authority and the public truth associated with it.

Scientific results can be ethically controversial because of their potential profound effect on society.