Can you name any evidence supporting the necessity of, to pick a moderately troublesome example off the top of my head, copyright? I'm not aware of any alternatives being tried (successfully or otherwise) in modern countries, so there's no actual evidence for its necessity. Shall we abolish governmental protection of intellectual property? That's a somewhat tenable position (donation-based profit, etc.), but I'm guessing most people here don't hold it.
I suspect that if your suggestion's consequences were carefully inspected, it would turn out to be more or less indistinguishable from a very extreme form of libertarianism. I'm aware of no clear evidence that prohibiting civilian possession of assault helicopters and anti-tank missiles is beneficial. Are you? Perhaps they'd be primarily used to resist oppressive governments.
It's also worth observing that plenty of professed rationalists take the exact opposite approach to you. They follow the precautionary principle: ban anything unless we have evidence it's not harmful.
I don't think much of either approach. Suggesting that we should have a hard-and-fast rule of what we have to do in the absence of clear evidence is a bad idea. Humans have capacities for intuition and logic in addition to our capacity to gather empirical evidence. If evidence is lacking, we need to take a best guess, not just say "let's permit/ban it".
Can you name any evidence supporting the necessity of, to pick a moderately troublesome example off the top of my head, copyright?
You seem to be expecting a much higher standard of evidence than I had in mind. Perhaps necessity was too strong of a word. Utility? Benefit? Something like that.
All I ask is that laws have 1) a clearly defined goal of solving a problem that society wants to solve, and 2) empirical evidence (gathered after the fact, if needed) that they are doing what they were intended to do with acceptable side-effects. Marijuana criminaliz...
When an LW contributor is in need of an example of something that (1) is plainly, uncontroversially (here on LW, at least) very wrong but (2) an otherwise reasonable person might get lured into believing by dint of inadequate epistemic hygiene, there seems to be only one example that everyone reaches for: belief in God. (Of course there are different sorts of god-belief, but I don't think that makes it count as more than one example.) Eliezer is particularly fond of this trope, but he's not alone.
How odd that there should be exactly one example. How convenient that there is one at all! How strange that there isn't more than one!
In the population at large (even the smarter parts of it) god-belief is sufficiently widespread that using it as a canonical example of irrationality would run the risk of annoying enough of your audience to be counterproductive. Not here, apparently. Perhaps we-here-on-LW are just better reasoners than everyone else ... but then, again, isn't it strange that there aren't a bunch of other popular beliefs that we've all seen through? In the realm of politics or economics, for instance, surely there ought to be some.
Also: it doesn't seem to me that I'm that a much better thinker than I was a few years ago when (alas) I was a theist; nor does it seem to me that everyone on LW is substantially better at thinking than I am; which makes it hard for me to believe that there's a certain level of rationality that almost everyone here has attained, and that makes theism vanishingly rare.
I offer the following uncomfortable conjecture: We all want to find (and advertise) things that our superior rationality has freed us from, or kept us free from. (Because the idea that Rationality Just Isn't That Great is disagreeable when one has invested time and/or effort and/or identity in rationality, and because we want to look impressive.) We observe our own atheism, and that everyone else here seems to be an atheist too, and not unnaturally we conclude that we've found such a thing. But in fact (I conjecture) LW is so full of atheists not only because atheism is more rational than theism (note for the avoidance of doubt: yes, I agree that atheism is more rational than theism, at least for people in our epistemic situation) but also because
Does any of this matter? I think it might, because
So. Is theism really a uniquely awful example? If so, then surely there must be insights aplenty to be had from seeing what makes it so unique. If not, though ... Anyone got any other examples of things just about everyone here has seen the folly of, even though they're widespread among otherwise-smart people? And, if not, what shall we do about it?