This graph shows the percentage of US academics that identify with various parts of the political spectrum. The graph for UK academics is pretty similar:
Two things are obvious from these graphs. The first is that there's now a big gap between the left and the rest in academia. The second is that this gap has increased over time.
An obvious question is: why? One possible answer is that left-wing people are just smarter, and so more of them make it as professors. Whether there's a link between certain political views and intelligence is still an open question. But in any case, if left-wingers have always been smarter, how come they've only come to dominate academia in the lat 20 years?
A second answer is, in the words of an academic friend I was talking to recently, 'The truth has a left-wing bias.' But does it? There's good scientific evidence for some typically left-wing ideas, such as climate change. But there's equally solid evidence for ideas that most left-wingers deny, for example that there are some natural sex differences.
If the truth doesn't really have a political bias, that shoots down the idea that right-wingers are less likely to get university posts because (for example) a creationist wouldn't make a good academic biologist. Someone who denies the evidence for sex differences probably wouldn't make a very good biologist either.
In fact, though, it's clear that the graphs above aren't a result of geography departments turning down flat-earthers in their job searches. Some of the most polarised parts of academia are fields in the humanities like English literature; one study suggested that only around 2% of English professors in the US are Republicans. But it's just not credible that you need to be left-wing in order to understand Shakespeare or Dickens.
So the idea that there are more left-wing professors simply because left-wing people are smarter or have a more accurate view of the world doesn't have much to say for itself. That leaves us with a more likely set of explanations involving selection - the process by which people sort themselves into groups.
One part of the selection that goes on in universities may be self-selection. Right-wing people may be more interested in making money and going into business, and left-wing people may be more interested in staying in public service, so that smart right-wing people select themselves out of academia, and smart left-wing people opt to stay in.
Another type of selection, though, may also be at work: discrimination. Right-wing academics perceive that they're being discriminated against, and left-wing academics admit that they would discriminate against conservatives in a hiring process. Conservative academics are also less likely to have academic jobs than their qualifications would predict.
If you're on Twitter or live in a university town, you might hear left-wing academics tell you that complaints about left-wing dominance of academia are just a conspiracy theory. Or that it's true there are more left-wing academics, but that it's just a natural consequence of left-wingers being smarter.
As we've seen, though, while there's very good evidence that left-wingers now dominate university teaching, there's no reason to believe that it's because right-wingers are stupider. What's more likely is that the current pattern is a result of a combination of two factors: right-wingers deciding to leave academia for their own reasons, and left-wingers discriminating against them.
Sam Butler is an English settler in New Zealand, a writer and an anti-intellectual. He tweets @ErewhonNam.