A fatal flaw of rationalism is that the spread of rational thinking often causes a net-decrease of rationality across the population. For every type of logical fallacy, for instance, there is a fallacy fallacy in which the name of the legitimate fallacy is invoked in a fallacious way. Occurrences of fallacy fallacy are now far more common—and more harmful—than the logical fallacies they supposedly discourage. Social awareness of the ad-hominem fallacy is used to prohibit Bayesian reasoning about the trustworthiness of a source, based on that source’s history and character. The dictum “correlation does not equal causation” increases the sophistication and confidence of those who deny real causal patterns, and so on. Beware, rationalists, you may just get what you ask for!