Is it always epistemically irrational to believe a conspiracy theory? Not on principle. According to the standard definition in the philosophical literature, conspiracy theories are too wide and heterogenous a class for us to proffer any universal rules re their rationality. This does not mean, however, that we cannot offer any useful generalisations. This paper argues that one useful generalisation concerns the contrariness of some conspiracy theories. Whilst we cannot say that it is always irrational to believe a conspiracy theory, ceteris paribus, it is irrational to believe a contrarian conspiracy theory. A conspiracy theory is contrarian when recognised credible epistemic authorities reject that theory. When a conspiracy theory is cont...