Proportionality Bias
Major Event–Major Cause Heuristic
The tendency to believe that big events must necessarily have big causes — a belief often seen in conspiracy-theory thinking as a justification for hidden coordination and groups.
Origin
Psychologists Patrick Leman and Marco Cinnirella provided key experimental evidence in a 2007 study linking the bias to conspiracy theory formation. Rob Brotherton brought the concept to a wider audience in his 2015 book Suspicious Minds: Why We Believe Conspiracy Theories, describing it as the human cognitive tendency to assume that large, significant events must have correspondingly large causes. The underlying pattern-seeking behavior has roots in broader attribution and causality research stretching back decades.
Everyday Use
A world leader is assassinated by a lone gunman, and people insist there must be a grand conspiracy — because a small cause feels too random for such a huge event. We apply the same logic to stock market crashes, celebrity deaths, and even traffic jams, assuming complexity must have an equally complex explanation.