Theism is often a default test of irrationality on Less Wrong, but I propose that global warming denial would make a much better candidate.
Theism is a symptom of excess compartmentalisation, of not realising that absence of evidence is evidence of absence, of belief in belief, of privileging the hypothesis, and similar failings. But these are not intrinsically huge problems. Indeed, someone with a mild case of theism can have the same anticipations as someone without, and update their evidence in the same way. If they have moved their belief beyond refutation, in theory it thus fails to constrain their anticipations at all; and often this is the case in practice.
Contrast that with someone who denies the existence of anthropogenic global warming (AGW). This has all the signs of hypothesis privileging, but also reeks of fake justification, motivated skepticism, massive overconfidence (if they are truly ignorant of the facts of the debate), and simply the raising of politics above rationality. If I knew someone was a global warming skeptic, then I would expect them to be wrong in their beliefs and their anticipations, and to refuse to update when evidence worked against them. I would expect their judgement to be much more impaired than a theist's.
Of course, reverse stupidity isn't intelligence: simply because one accepts AGW, doesn't make one more rational. I work in England, in a university environment, so my acceptance of AGW is the default position and not a sign of rationality. But if someone is in a milieu that discouraged belief in AGW (one stereotype being heavily Republican areas of the US) and has risen above this, then kudos to them: their acceptance of AGW is indeed a sign of rationality.
Here's the main thing that bothers me about this debate. There's a set of many different questions involving the degree of past and current warming, the degree to which such warming should be attributed to humans, the degree to which future emissions would cause more warming, the degree to which future emissions will happen given different assumptions, what good and bad effects future warming can be expected to have at different times and given what assumptions (specifically, what probability we should assign to catastrophic and even existential-risk damage), what policies will mitigate the problem how much and at what cost, how important the problem is relative to other problems, what ethical theory to use when deciding whether a policy is good or bad, and how much trust we should put in different aspects of the process that produced the standard answers to these questions and alternatives to the standard answers. These are questions that empirical evidence, theory, and scientific authority bear on to different degrees, and a LessWronger ought to separate them out as a matter of habit, and yet even here some vague combination of all these questions tends to get mashed together into a vague question of whether to believe "the global warming consensus" or "the pro-global warming side", to the point where when Stuart says some class of people is more irrational than theists, I have no idea if he's talking about me. If the original post had said something like, "everyone whose median estimate of climate sensitivity to doubled CO2 is lower than 2 degrees Celsius is more irrational than theists", I might still complain about it falling afoul of anti-politics norms, but at least it would help create the impression that the debate was about ideas rather than tribes.
The irrational thing, and I see it often, is people who believe "nothing should be done about global warming and therefore at least one of the questions above has an answer of 'none'". Obviously, they don't use those words. But when someone switches freely from "the earth isn't getting warmer" to "the fact that the earth is getting warmer is part of a natural climate cycle" there's something wrong with that person's thinking.