Beware Change-Blocking Priors

“Faced with the choice between changing one’s mind and proving that there is no need to do so, almost everyone gets busy on the proof.” John Kenneth GalbraithWhen trying to resist evidence offered for contrarian claims, many invoke the mantra: “extraordinary claims require extraordinary evidence”. Sometimes they just mean to point to Bayes’ Rule, where posterior beliefs are renormalized products of prior beliefs and evidence likelihoods, so that unusually low priors must be countered by unusually strong evidence.But at other times, those who invoke this mantra point instead to how many big implications a claim would have if true, for personal and social practices, and for what else we believe. For example, this has been invoked by skeptics of cold fusion, room temperature superconductors, COVID lab leak, high-intensity interval training, low marginal medical effectiveness, big battery breakthroughs, cryonics, life on Mars, and UFOs as aliens. In the past, such arguments have been…