I haven't spent as much time studying the global warming debate as I have studying the creation-evolution debate, but to the extent I have studied it, the evidence seems uniformly supportive of the view that human activity has caused significant warming.
I'll note that most of the resistance to global warming that I've seen on LW is not on these points. (Bias note: I think my position is roughly central of global warming resistance I've seen on LW, but if I err in describing opinion on LW it will be by pushing the opinion too far towards myself.) The evidence seems very solid that human activity has altered the atmospheric concentrations of CO2, and that this results in higher temperatures.
The primary issues are:
The question of which temperature / global climate is optimal is rarely discussed, suggesting that the current climate is unlikely to be optimal.
Agree that this topic is not widely discussed in mainstream climate science as such, but not sure your conclusion follows.
The chances of the current global climate being optimal would be pretty remote, except for the facts that one very powerful (but slow) optimisation process has been busy adapting the existing biota (including humans) to the climate, followed by an even more powerful and much more rapid optimisation process adapting human capital and expertise to the existing climate.
It's not that we can't adapt to another set of climate and weather patterns, it's that it'll have a cost. There certainly is a reasonable amount of discussion in the mainstream climate literature about the likely costs of adaptation. Which, of course, one needs to weigh against the likely benefits.
Predicting the local weather impact of global climate change is fearsomely difficult and uncertain (far more so than global temperature), which makes adding up the cost/benefit analysis extremely difficult. (And makes planning long-term capital investments difficult.)
It seems to me pretty likely that the current global climate is at least a local optimum for humans.
Climate science has incentives that seem unusually pointed away from truth-discovery compared to other fields, and there seems to be evidence of climate scientists (rationally!) responding to those incentives.
Really? What kind of incentives?
This isn't an issue I care much about, so I hope you won't mind a quick treatment. The 5-second version is that more is on the line for the researchers personally with climate change research than other sorts of science, that scientific reports have consequences for politics which forces the scientists to be more involved in politics than other sorts of scientists, and that this corroding influence of politics adds friction to the processes that pump errors out of science. Here's an incomplete list with more details:
The most visible one is media pressure, which rewards more shocking possibilities. (Did you ever hear about clathrate gun hypothesis? Did you hear that further research didn't pan out?) In most other fields, the media rarely if ever comes calling, and so more of the incentives come from other researchers. (I remember a historian, who was called up because Sarah Palin had made a correct but non-obvious comment about Paul Revere, who pointed out that he had never been interviewed until Palin had mentioned Revere.)
Relatedly, the funding and visibility of climatology depends in large part on how much people care about climatology. This is true for many fields- NASA seems t...
Climate change / global warming is real and man-made. It will come as a big surprise that climate change from 1900 to 2025 has mostly been a net benefit, rising to increase welfare about 1.5% of GDP per year. Why? Because global warming has mixed effects and for moderate warming, the benefits prevail. The increased level of CO₂ has boosted agriculture because it works as a fertilizer and makes up the biggest positive impact at 0.8% of GDP. Likewise, moderate warming avoids more cold deaths than it incurs extra heat deaths. It also reduces the demand for heating more than increases the costs of cooling, totaling about 0.4%. On the other hand, warming increases water stress at about 0.2% and negatively impact ecosystems like wetlands at about 0.1%. Storm impacts are very small, as the total storm damages (including naturally caused storms) are about 0.2%.
As temperatures rise, the costs will rise and the benefits decline, leading to a dramatic reduction in net benefits. After year 2070, global warming will become a net cost to the world, justifying cost-effective climate action.
The problem with saying "I'm only talking about X, I'm not talking about Y" when Y is related to X but less extreme, is that in politics people who are Y are often caricatured as being X. It's pretty hard to tell the difference between someone who really means to attack X and only X, and someone who is attacking Y by implicitly accusing them of being X and then attacking X.
It's the same reason as to why Jews might feel themselves to be a target when someone argues how bad it is to kill Christian babies to use their blood to bake matzohs, even if they have not personally killed any babies. Certainly if anyone posted such an argument I'd mod them down, even if I agreed that it's bad to eat babies and that they have correctly stated why.
Another issue: it may seem surprising that I haven't been able to uncover clear evidence of a field corrupted by political ideology, the way philosophy of religion and Biblical scholarship are significantly corrupted by religious agendas.
"Women's studies"? (I'm sure they're pretty much spot-on about many things, but it's also seems like the kind of environment where people can go off the deep end.)
In my personal contact with this crowd (women's studies, not sociology, to be clear), I've often been quite frustrated. When approached in a reasonable manner, at least those I've encountered have been reasonable in return, willing to seriously discuss problems and alternative theories. But in their public statements and publications they go to great lengths to avoid expressing any doubt or disagreement with one another or with the orthodoxy in the field. I suppose they feel solidarity is necessary in the face of outside threats, and I'm not unaware that they do in fact face outside threats, but this still seems to me like a problem.
Howard Zinn
I don't think using Zinn as an example of objective work is going to work well. Many historians have criticized Zinn. One of the more disturbing criticisms is a complete failure to update known issues when going to later editions of his book. Loewen seems like a substantially better example in this context.
I think there's a wrench in the historical Jesus example. While a majority of Biblical scholars agree that he existed, there's almost no consensus about who he was, which seems to be the driving force behind those who are agnostic or skeptical about Jesus' existence. In other words, there's a consensus about Jesus' historicity but there's no consensus about why he was actually important to early Christians. Which is strange.
As a matter of fact, there's a huge halo effect bias among Biblical scholars when describing Jesus in a supposed secular academic con...
I think your point on the lack of consensus in philosophy is somewhat unfair. A lot of questions in the survey were chosen because they were hard or unsolved problems. A similar survey of unsolved hard questions in physics would also produce split answers. (for example ask if there actually an island of stability at higher atomic numbers).
On the easy questions, consensus seemed to be 95%+ once you threw out the 14% of theists (assume them to choose the answer logically required by theism) and people choosing "other".
"That means all a non-expert needs to do is look at the data on expert opinion and if there's no strong majority agreement, they should feel pretty comfortable ignoring the experts when they feel it's necessary to do so."
If experts are divided on a yes-no question, you may have a sizable chance of being right if you go with the minority view. But that does not mean that the strategy of going with your personal judgment will outperform the strategy of going with modal or plurality expert opinion (or systematically transformed opinion).
Compare and contrast a same-day post: http://lesswrong.com/r/discussion/lw/iuc/link_distance_from_harvard/
A little editorial nitpick.
Among US historians, there's an obvious counter-point that people like Howard Zinn and James W. Loewen don't at all unusual.
Is there a verb like "seem" missing between "don't" & "at"?
Bit of an aside, but have you read Reza Aslan's Zealot, and if so what are your thoughts? Reza Aslan is another interesting point here as he doubted his faith during his studies, although instead of becoming agnostic he reverted back to Islam.
However I brought up the book more because it provides a middle way out. Aslan's historical Jesus is someone very different from the Christ figure, whose existence seems more plausible than fabrication given the scale involved. There was, after all, an active cult of his followers in Jerusalem for decades after his de...
There's a problem if two groups of experts claim responsibility for the same question and answer it differently. The example I have in mind is doctors vs. psychotherapists on ADHD, but I'm sure there are many others.
It's easy, but not helpful, to use "postmodern" as a shorthand for "bad ideas" of some kind. Something like Sturgeon's law ("90% of everything is crap") applies to postmodernism as to everything else, and I'd even agree that it's a kind of thinking that is more likely than average to come unmoored from reality, but that doesn't mean that it's barren of all insight. Especially today, at least 20 years after its heydey, and considering that even in its heyday it was a very rare academic department indeed where drinking the kool-...
my impression is that even ignoring head counts the historicists have the better of the argument.
Why? I haven't looked into it much, but it looks to me like they've ignored alternative hypotheses (and that they met with mythicism first in the form of crack-pottery). Can you point me to about 2 decibels of evidence you don't think I've seen?
Also, this looks like a mistake:
possible Carrier has some decisive decisive evidence
This seems like a Procrustean-bed attempt at forcing disparate areas of human endeavor into a single reference class of "fields of expertise" in order to use an outside view to evaluate the correlation between expert consensus and trustworthiness. It is understandably tempting, since, if successful, it would give an outsider a tool to evaluate the reliability of the experts' statements. Unfortunately, these "fields of expertise" are vastly different. Some use the scientific method, like biology and environmental sciences, and maybe econ...
I recently read Bryan Caplan's The Myth Of The Rational Voter. It's an excellent book in a lot of ways, and one of those ways is how it got me thinking about the issue of expert consensus. It's pushed me more towards thinking that hard data on what the experts in a given field believe about their area of expertise is incredibly useful. Specifically, based on examples from a variety of fields (listed below the fold), I'll conclude:
Expert opinion plays a central role in Caplan's Myth of the Rational Voter. Caplan argues that voters aren't just ill-informed about many topics, they suffer from biases that lead them to have views that systematically differ from those of the experts. And Caplan focuses on economics, both because it's his own area of expertise and because of it's obvious relevance for policy making.
I've heard The Myth of the Rational Voter described as having a "libertarian" perspective, but while Caplan is himself a libertarian, that didn't strike me as coming across particularly strongly in the book. I say this as someone who considers himself a fan of Paul Krugman, and in fact Caplan frequently cites Krugman in support of his points.
Furthermore, Caplan has hard data to back his claims up: one study found that, in 2000, 72.5% of economists mainly agreed with the claim that "tariffs and import quotas usually reduce the general welfare of society." A further 20.1% said they "agreed with provisos," while only 6% "generally disagreed." In another example Caplan cites, nearly three-quarters of economists generally disagreed with the statement "wage-price controls should be used to control inflation."
However, these numbers are not as overwhelming as the numbers on scientific support for evolution. And Caplan does not mind sometimes rejecting the majority view among his colleagues. He writes:
If my premise is that the economic consensus is reliable, how can I reach a conclusion that the economic consensus rejects?...
This complaint would be airtight if my premise were that the economic consensus is infallible. But my actual premise is merely that economists, like other experts, deserve the benefit of the doubt—and that the burden of proof rests on those who would question the expert consensus. Since the rational voter assumption is part of that consensus, my responsibility as a naysayer is to refute it—which is precisely why I needed to wrote this book.
I take it there's a connection between the fact that consensus in economics tends to be weaker, and Caplan's willingness to contradict the consensus on some issues. As a majority gets smaller, the chances that a scholar with the minority position might have good reasons to reject the minority view gets larger. Furthermore, when the majority view only has around 70%-75% of experts in its favor, it's also much more plausible that an informed layman could have good reasons to reject the majority view. Even though I'm not an economist, I don't hesitate to think Caplan is probably right to reject the rational voter assumption. Still, non-experts who have who know little about a debate should generally be willing to accept the conclusions of a 70+% majority.
Also relevant: Krugman himself has written about the issue of consensus in economics. His conclusion is that "most of what economists do is indeed fairly objective and non-ideological," and can produce considerable agreement among economists. However, there are some important issues in business-cycle macroeconomics where there are big, ideologically-driven disagreements. Even so, he notes that in one panel of 42 economists from top schools, 80% agreed with the statement that the 2009 American Reinvestment and Recovery Act "significantly boosted output and employment."
Something I almost forgot to mention, because it seems obvious to me, but probably doesn't seem obvious to other people: the views of economists probably can't be explained away as driven by political ideology. Anecdotally, people like Krugman are a problem for attempts to dismiss economists as libertarian ideologues (though I guess some people would dismiss Krugman as a neo-liberal sellout).
Furthermore, Caplan argues that data from the Survey of Americans and Economists on the Economy suggests the opinions of economists mostly cannot be explained as the result of ideology or socioeconomic class. He notes that, in spite of occasional accusations of "free-market fundamentalism" hurled at economists, the reality is that economists are acutely aware of the weaknesses of markets.
This is my own field, as much as someone who's left academia can claim a field as his own. And in philosophy, we have unusually good data about what professional philosophers believe about the major disputes in their field, thanks to a 2009 survey organized by philosophers David Bourget and David Chalmers through the website PhilPapers.org.
Robin Hanson commented when the survey data was released, and seemed happy to learn he agreed with the modal expert opinion on 25 out of the 30 questions surveyed. But this seemed to me, and still seems to me, the wrong reaction, given that on most questions there was no majority view. On the questions where there was a majority view, on about half of those there was still a large amount of disagreement, with less than 60% of philosophers accepting the majority view.
Under such circumstances, I'm inclined to think the opinion of the experts tells us very little about what the right view is. My reaction was partly colored by experience within philosophy, which has led me to think most philosophers don't have very good reasons for their philosophical opinions and agnosticism towards particular questions is more often the right view than most philosophers would admit.
It's interesting, though, to look at look at the three questions where there was most agreement. 81.6% of philosophers endorse non-skeptical realism about the external world, 75.1% endorse scientific realism, and 72.8% endorse atheism. Note that on all questions, there was a significant number of "other" answers, for example, on God, only 14.6% of philosophers endorsed theism, 5.5% were agnostic, while others endorsed various idiosyncratic views.
My initial reaction to these numbers were that they were the exceptions that proved the rule. "Non-skeptical realism about the external world," is basically just the claim that there's stuff (tables and chairs and trees and rocks and so on, or perhaps subatomic particles) that exists outside our minds and we can know about them. The fact that only about 80% of philosophers are able to agree on this seemingly commonsensical proposition just underlines how incapable of agreeing about anything philosophers are.
Scientific realism, similarly, is just the view that science describes the real world, and the less-than-total agreement of philosophers on this idea is similarly disappointing. The question of God's existence, meanwhile, strikes me as having more in common with the question of whether fairies exist than most other questions on the survey.
Yet as I've thought about the question more, particularly philosopher's opinions on God's existence, I've shifted towards thinking there are some questions where a strong majority of philosophers agreeing is significant. For one thing, it's striking how few theistic philosophers are interested in defending traditional arguments for the existence of God. With a few exceptions (most notably Richard Swinburne and William Lane Craig), they stick to arguing that belief in God is reasonable. That certainly seems worth mentioning if you're an atheist trying to persuade theists that there are no good arguments for the existence of God.
Reading The Myth of the Rational Voter caused my views to shift further, because it made me realize the level of agreement you'll find among philosophers on the external world, science, and God is about as high as you'll find among economists on major questions of economics. This makes me think that probably atheists should not be shy citing strong support for atheism among philosophers as a reason to be an atheist. I wouldn't claim it's conclusive all by itself, but it does create a presumption that atheism is the most rational view, which should place a strong burden of proof on the theist to overcome.
Arguably, there's also a lesson here for philosophy professors teaching undergrads (particularly in 101 classes). The standard approach to teaching philosophy is to make a show of being very even-handed in presenting any given debate, at most subtly prodding students towards the professor's preferred view. But it might be better for professors to be unapologetic about explaining to students that most philosophers are atheists and why.
The value of taking the same approach to knowledge of the external world and science may be less obvious, but I suspect it may be even more important. Too many undergraduates come out of their philosophy 101 classes thinking the lesson of philosophy is that we don't really know anything, or that science is a matter of faith too. Philosophy professors should consider trying to actively fight that tendency.
(What about the other issues that had relatively high levels agreement among philosophers? Professors might make a point of defending them too, though the next four most agreed-upon points from the PhilPapers survey aren't as exciting: the a priori, the analytic/synthetic distinction, the trolley problem, and cognitivism about moral judgement. At least, they're not as exciting until you connect them to other, more controversial, issues.)
The broader lesson I take away from this is that you don't need to worry about looking for the kind of symptoms that Luke has cited in diagnosing philosophy as a "diseased discipline." The example of philosophy suggests that when a group of experts is deeply confused about the subject-matter of their own discipline, they mostly fail to converge on any kind of strong majority agreement. That means all a non-expert needs to do is look at the data on expert opinion and if there's no strong majority agreement, they should feel pretty comfortable ignoring the experts when they feel it's necessary to do so.
And yes, though I'm inclined towards agnosticism on a lot of philosophical questions, that does mean it's often OK for non-philosophers to have strong opinions on philosophical questions that philosophers themselves can't agree on. For example, I think Eliezer should feel pretty comfortable with his advocacy of one-boxing on Newcomb's problem, even though a plurality of philosophers support two-boxing.
There's an important side-note to make here regarding the opinions of specialists in philosophy of religion. They, unlike professional philosophy as a whole, tend to be theists; 72.3% theists vs. 19.1% atheists. Some theistic philosophy-bloggers took this as vindication of theism, arguing philosophy of religion specialists know the most about the issues so they're who we should trust. On the other hand, it looks suspicious that they haven't managed to convince their colleagues in other specialties. What should we make of this?
The answer becomes obvious once you know a little more about the professional dynamics surrounding philosophy of religion. Philosophy of religion is not taken very seriously in mainstream philosophy. That is not a controversial point; philosophers of religion know it and complain about it. When I was at Notre Dame (the department rated number one for philosophy of religion in the world), one of the department's philosophers of religion specifically had the job of taking the religious grad students aside and telling them they had to specialize in something other than philosophy of religion or else they would not get a job.
This means atheist philosophers have basically no incentive to work in philosophy of religion. If you read the biographies of most philosophers of religion, overwhelmingly they were religious before they started studying philosophy and got into philosophy of religion because they really, really wanted to use philosophy to defend and/or elaborate on their religious beliefs. Even philosophers of religion are beginning to notice that philosophy of religion often resembles religious apologetics more than objective inquiry.
I'll save the technical account of why this matters for the section on Biblical scholarship, but here I'll note that it's hard to explain away mainstream support for atheism in a parallel manner. Academics do tend to be less religious than the general population, true, but philosophers are an outlier even among other academics. A 2009 study that looked at religious belief within the 20 largest academic disciplines found that only about 10 percent of academics are atheists, with psychologists being the most atheistic at 50 percent. 50 percent is extremely high compared to the general US population, but still not up to philosopher levels.
I unfortunately don't have good data on the opinions of Biblical scholars, but I'm going to talk about it because it's an area I know a lot about (particularly historical Jesus research), and there's an important point to be made here about how non-experts can decide whether the opinions of experts are trustworthy.
The point is a rather obvious one: Christian apologists like William Lane Craig often cite the (alleged) consensus of Biblical scholarship in support of their arguments, but even if the basic claims were true, it wouldn't be to find that Biblical scholars tend to have views favorable to Christianity (and Judaism) because Biblical scholars are overwhelmingly Christians and Jews. Biblical scholars tend to get their degrees from seminaries rather than secular academic departments. Atheist Biblical scholar Jacques Berlinerblau once estimated that at a typical meeting of a thousand members of the Society for Biblical Literature, you'd find only a couple dozen atheists (I'm not sure if he meant to include people like Bart Ehrman, who identifies as an agnostic).
Non-believing Biblical scholars tend to be former believers who deconverted in part due to their studies; this is true of Bart Ehrman, Gerd Lüdemann, Michael Goulder, and Robert M. Price. Jacques Berlinerblau is an exception to the "former believers" rule. Hector Avalos is also a partial exception (his interest in the subject stems from his time a teen evangelist, but he deconverted before starting his graduate studies). I e-mailed Berlinerblau about this once, and he could think of one other scholar in the field who he didn't think had ever been a believer, but confirmed that such people are extraordinarily rare.
This doesn't mean that outside the few dozen atheists and agnostics, Biblical scholars are all going to agree with William Lane Craig, because there are also plenty of liberal Christians like John Dominic Crossan and Marcus Borg in Biblical scholarship. It does mean that if it did turn out that most Biblical scholars had views favorable to (traditional) Christianity, it wouldn't mean much because it would be no more surprising that finding out that Muslim scholars tend to have views favorable to Islam. It's the Crossans and the Borgs that are surprising (regardless of the exact liberal Christian / conservative Christian breakdown).
This common-sense argument in terms of what is and isn't surprising is reinforced by a Bayesian analysis: if the opinions of a group of experts can be predicted using information that isn't relevant to whether or not their opinions are true, knowing what their opinions are gives you no information on the claims in question. It's important to distinguish this heuristic from post hoc explaining away of expert opinions you don't like: conservative Christians often propose unflattering explanations for the opinions of the Crossans and the Ehrmans, but it's unlikely that they could have predicted their opinions using information not relevant to whether their opinions are right.
So far, I probably haven't said anything that would surprise the average member of LessWrong: of course Jesus didn't perform any miracles or rise from the dead. So let's tackle a slightly harder question: was there a historical Jesus at all? Or, to put it somewhat more precisely: can Christianity be traced back to the followers of a single Jewish preacher who was executed by the Roman authorities in Palestine in the first half of the first century A.D.?
That Christian scholars think Jesus existed isn't surprising. Even Crossan has his own version of the historical Jesus, as a wise teacher who's palatable to liberal churchgoers, that he wants to sell. Knowing the opinions of Crossan et al. may be slightly informative, since since many liberal Christian and Jewish scholars seem to have reconciled themselves to there being no historical Moses, so the fact that they haven't gone that route with Jesus may give us some information. But looking at the opinions of non-believing scholars specifically will be more informative.
Though we don't have survey data because non-believing Biblical scholars are so rare, and scholars who doubt Jesus' historicity even rarer, we can make an estimate. Richard Carrier (a historical Jesus skeptic with a PhD in history from Columbia University) recently listed Arthur Droge, Kurt Noll, and Thomas Thompson as people who "agree historicity agnosticism is warranted," in addition to listing Thomas Brodie, Robert M. Price, and himself as being "even more certain historicity is doubtful." Because historicity skeptics are so rare, and because Carrier knows the debate so well, this list is probably exhaustive at least as far as publicly expressed views are concerned.
There are a number of difficulties here. First, it's unclear whether Berlinerblau's "two dozen" estimate includes agnostics. Second, categorizing Carrier's list of skeptics gets tricky in a few cases. Robert M. Price is a non-believer who's officially agnostic on whether Jesus existed, but he's written a lot about the issue and is quite confident agnosticism is the right view, justifying his inclusion on the "more certain" list. Brodie has apparently tried to argue he can be a good Catholic while rejecting the idea of a historical Jesus (which, as I've already noted, is a position some people have already taken on Moses). Noll describes himself as "theistic off the job and professionally agnostic."
Finally, Brodie has apparently been convinced there was no historical Jesus since the 1970s, but until recently was keeping quiet about it (for fear of the professional repercussions?) That raises the question of whether there might be other skeptics out there still in the closet. Still, I'd venture that probably at least 75% (Carrier's 6 divided by Berlinerblau's 24) of non-religious scholars with relevant expertise are at least fairly confident Jesus existed. Much of the remainder would be agnostic rather than full-blown mythicists. That would be strong, but not overwhelming, support for historicity.
Having read both defenses of historicity by people like Ehrman, and critiques by Robert M. Price and Earl Doherty (who's an amateur, but has Carrier's endorsement), my impression is that even ignoring head counts the historicists have the better of the argument. But Carrier has said he doesn't expect most scholars to agree with him without having seen his research and arguments. He'll be presenting those in a book which is slated for release early next year. I think the book probably won't persuade many people, but I don't think Carrier's approach is necessarily foolish. It's possible Carrier has some decisive evidence that other people just don't have access to, yet.
Reference to global warming on LessWrong sometimes elicit negative reactions (often with references to a misinterpreted version of Politics is the Mind-Killer), yet on the merits global warming seems to be in the same situation as evolution. Wikipedia, again, has a good summary of the data on expert opinion, even better than the one for evolution. Highlights include:
The above examples largely exhaust the cases I know about where a strong majority of experts agree on an issue, yet the issue is controversial among people outside the field. The age of the earth, the age of the universe, and the Big Bang are also technically examples because young earth creationism, but you already knew that.
However, I can think of some minor examples that seem to support the principle that when there's little to no agreement among the experts, non-experts should focus on other evidence if they're going to have an opinion. For example, there's currently no majority view among physicists on how to interpret quantum mechanics, and nothing strikes me as inherently foolish about Eliezer's advocacy of many-worlds. Similarly, though I don't have data my impression is that evolutionary biologists and psychologists are divided on evolutionary psychology, but I feel no hesitation in thinking that the basic tenets of evolutionary psychology are correct.
Can we find counter-examples to the heuristics suggested above? A case where, for non-obvious reasons, a strong majority of the experts in some field have converged on a crazy conclusion? The more heavily postmodern corners of academia might be a good place to look for that. In "The Myth of the Postmodern University," Brian Leiter argues that the influence of postmodernism on academia has been greatly exaggerated, but it has had significant impact on English, literature, and history. Leiter also quotes a reader e-mail suggesting that communication, media studies, cultural studies, and journalism may also belong on the of fields heavily influenced by postmodernism. So it may be worth looking at those.
But my guess is that there may be less there than meets the eye. I don't doubt you can find many people with crazy pomo ideas in those fields, but I suspect you'd also find lots of people in those same fields who generally reject the crazy pomo ideas. At worst, I'd expect to find 50-odd percent support for the crazy view with a significant number of objectors. But I may be wrong about this, and it would be helpful to find good data.
Another issue: it may seem surprising that I haven't been able to uncover clear evidence of a field corrupted by political ideology, the way philosophy of religion and Biblical scholarship are significantly corrupted by religious agendas. This may be because religions like Christianity have a fairly definite body of doctrine (left-wing theology aside), making bias easy to detect. Political categories like "liberal" or "conservative," on the other hand, are squishier, which plausibly makes bias harder to detect.
Indeed, though the effects may be less obvious, there does seem to be evidence for a general self-selection effect: liberal college students are more likely to pursue careers in academia. That certainly suggests you shouldn't vote for Democrats just because most college professors do, but it's unclear how it affects specific academic debates. The fact that even liberal economists tend to have many views that don't fit the liberal stereotype suggests these effects tend to get swamped by the effects of having relevant expertise, when someone's dealing with their own field. But perhaps there are other, less encouraging examples out there that I'm unaware of.
Finally, left-right political bias may be much less important than nationalistic bias. I once heard it said (by Orwell? Russell? Google isn't helping me find the quote) that if you're from a small country that hasn't played an important role in world affairs, you'll likely be able to find an objective history of your country written by a foreign historian, but otherwise you're probably out of luck for an objective history of your country. Among US historians, there's an obvious counter-point that people like Howard Zinn and James W. Loewen don't seem at all unusual. Which is not to say they're necessary very objective, just that the biases among US historians may run in enough different directions to prevent the entire history profession from converging on nonsense.
On the other hand, even among left-wing historians in the US, how many would go as far as gwern does and say the American Revolution was a mistake? Yet gwern seems to to have a strong case: it's definitely a bad thing when a war kills tens of thousands of people, and the US and Canada didn't turn out all that different. So, contrary to what many would expect, it may be that even left-wing US historians are too positive about America. This suggests that, on issues where nationalistic bias could be a problem, you should be cautious about accepting the majority opinion of historians unless you have international data.
For some extra context for this post, see my discussion post on academic cliques.