I'll give three examples.
Yesterday Tom trashed the ridiculous argument that the Tea Party movement is similar to that of Ross Perot supporters in the 1990s. The argument is ridiculous because, as Dan Balz explains, Perot's was a movement of the political center, whereas the tea partiers are closely tied to the right wing of the Republican Party.
How could people be confused on this point? My theory (which I'd be happy for a cognitive psychologist to look into further) is that it has just the right level of complexity to seem as if it could be right. Both the Tea Party and the Perot supporters have talked about deficits, they both seem like movements of outsiders, so people draw the easy connection without thinking further.
Another example is the familiar but oft-refuted claim that political polarization is caused by gerrymandering of congressional districts. This certainly sounds like it could be true: Congress really is more polarized than it used to be, and there really is gerrymandering. The evidence doesn't support it, but the idea doesn't die--it has just the right level of complexity to be plausible.
And here's another example:
John Lanchester writes in the London Review of Books:
Labour have an enormous statistical advantage going into the election. The simple way of putting this is to say that votes in the country are worth less than votes in the city. That's because the Boundary Commission has struggled to keep up with the historic drift of Britons out of cities into the country . . . Country constituencies are bigger, in population as well as geographical terms, than urban ones . . . Because Labour's support skews urban and the Conservatives' skews rural, this translates into a big advantage for Labour. How big? Well, this non-partisan article from the House of Commons magazine, dating from 2006 when the election was a long way off, reckoned that the Tories needed to win the election by a margin of 10 per cent in order to have any majority at all.
If you follow the link, though, it appears that (a) the boundaries were redrawn in 2005 or 2006, so the boundaries are only 4 or 5 years out of date, and (b) the "10 percent majority" thing is not coming from any imbalances in district sizes:
The Conservatives will still need a swing of about 10 per cent to win power outright . . . A swing of just over one per cent will now cost Labour its overall majority, compared to 1.8 per cent with boundaries unchanged. The changes reduce the swing needed by David Cameron to secure an overall majority from 11 per cent under the old boundaries to nine or 10 per cent with the constituencies which will be used in the next general election . . .
Got that? The effect of redrawing the district lines is estimated to be something like 0.7% of the vote, not 10%. (I'm getting 0.7% by subtracting "just over one per cent" from "1.8 per cent.") If the boundary change in 2005 or 2006 comes to the equivalent of 0.7% of the vote, then I'd expect any population shifts since then to account for less than 0.7% in partisan advantage.
Less than 0.7%. Not 10%.
The 10% is coming from the multiparty system, which has at times benefited the Labour party and at times benefited the Conservatives and doesn't seem to have benefited the Liberal Democrats at all (yet).
Again, what we have is a superficially plausible argument that goes in the right direction, and so it's easy to miss the details and go with it.
P.S. How does this post relate to the usual 538.com theme of giving you new information about politics? The connection is that we often talk about how we're right and the conventional wisdom is wrong. But when you make such a claim, it's a good idea to think occasionally about how it is that wrong ideas have such staying power. I think there's something appealing about reasonable-seeming ideas that have a certain level of technical complexity.
P.P.S Commenter Andy writes that the U.K. boundaries are based on census data from 2001, not 2005. This does not affect my main argument--the population imbalance still cannot be causing an effect even close to what is happening because of the three-party system--but I'm happy to be corrected. But the best comment of all is Grrigg's.