“The facts have a well-known liberal bias,” declared Rob Corddry way back in 2004 — and experience keeps vindicating his joke. But why?

Not long ago Ezra Klein cited research showing that both liberals and conservatives are subject to strong tribal bias — presented with evidence, they see what they want to see. I then wrote that this poses a puzzle, because in practice liberals don’t engage in the kind of mass rejections of evidence that conservatives do. The inevitable response was a torrent of angry responses and claims that liberals do too reject facts — but none of the claims measured up.

Just to be clear: Yes, you can find examples where *some* liberals got off on a hobbyhorse of one kind or another, or where the liberal conventional wisdom turned out wrong. But you don’t see the kind of lockstep rejection of evidence that we see over and over again on the right. Where is the liberal equivalent of the near-uniform conservative rejection of climate science, or the refusal to admit that Obamacare is in fact reaching a lot of previously uninsured Americans?

What I tried to suggest, but maybe didn’t say clearly, is that the most likely answer lies not so much in the character of individual liberals versus that of individual conservatives, as in the difference between the two sides’ goals and institutions. And Jonathan Chait’s recent thoughts on the inherently partisan nature of “data-based” journalism are, I think, helpful in bringing this better into focus.

As Chait says, the big Obamacare comeback and the reaction of the right are a very good illustration of the forces at work.

The basic facts here are that after a very slow start due to the healthcare.gov debacle, almost everything has gone right for reform. A huge surge of enrollments more than made up the initially lost ground; the age mix of enrollees has improved; multiple independent surveys have found a substantial drop in the number of Americans without health insurance.

Opponents of Obamacare could respond to these facts by arguing that the whole thing is nonetheless a bad idea, or they could accept that the rollout has gone OK but call for major changes in the program looking forward. What they’re actually engaged in, however, is mass denial and conspiracy theorizing strongly reminiscent of their reaction to polls showing Mitt Romney on the way to defeat, or for that matter evidence of climate change. Acceptance of the facts is, well, unacceptable.

Nothing illustrated this better than the reaction to Ezra Klein’s own note about the resignation of Kathleen Sebelius, which was intended as analysis rather than advocacy; Klein simply made the fairly obvious point that the HHS secretary was in effect free to resign now because Obamacare has been turned around and is going well. But Klein’s statement was met with a mix of outrage and ridicule on the right; how dare he suggest that the program was succeeding?

Why is it, then, that the right treats statements of fact as proof of liberal bias?

Chait’s answer, which I agree is part of the story, is that the liberal and conservative movements are not at all symmetric in their goals. Conservatives want smaller government as an end in itself; liberals don’t seek bigger government per se — they want government to achieve certain things, which is quite different. You’ll never see liberals boasting about raising the share of government spending in GDP the way conservatives talk proudly about bringing that share down. Because liberals want government to accomplish something, they want to know whether government programs are actually working; because conservatives don’t want the government doing anything except defense and law enforcement, they aren’t really interested in evidence about success or failure. True, they may seize on alleged evidence of failure to reinforce their case, but it’s about political strategy, not genuine interest in the facts.

One side consequence of this great divide, by the way, is the way conservatives project their own style onto their opponents — insisting that climate researchers are just trying to rationalize government intervention, that liberals like trains because they destroy individualism.

But this can’t be the whole story. It doesn’t explain, for example, the rejection of polls in 2012, or the refusal of the right to admit that things weren’t going well in Iraq — both cases in which conservatives really did have an interest in the outcomes. So what else differentiates the two sides?

Well, surely another factor is the lack of a comprehensive liberal media environment comparable to the closed conservative universe. If you lean right, you can swaddle yourself 24/7 in Fox News and talk radio, never hearing anything that disturbs your preconceptions. (If you were getting your “news” from Fox, you were told that the hugely encouraging Rand survey was nothing but bad news for Obamacare.) If you lean left, you might watch MSNBC, but the allegedly liberal network at least tries to make a distinction between news and opinion — and if you watch in the morning, what you get is right-wing conspiracy theorizing more or less indistinguishable from Fox.

Yet another factor may be the different incentives of opinion leaders, which in turn go back to the huge difference in resources. Strange to say, there are more conservative than liberal billionaires, and it shows in think-tank funding. As a result, I like to say that there are three kinds of economists: Liberal professional economists, conservative professional economists, and professional conservative economists. The other box isn’t entirely empty, but there just isn’t enough money on the left to close the hack gap.

Finally, I do believe that there is a difference in temperament between the sides. I know that it doesn’t show up in the experiments done so far, which show liberals and conservatives more or less equally inclined to misread facts in a tribal way. But such experiments may not be enough like real life to capture the true differences — although I’d be the first to admit that I don’t have solid evidence for that claim. I am, after all, a liberal.