[I’ve created this post as a public service, because there are few places on the Web where this fallacy is recorded, and the places where it is strike me as liable to linkrot.]
A “Wykehamist” is someone who went to Winchester College. The Wykehamist Fallacy has been a source of some terrible errors in Western foreign policy, so it’s a shame it isn’t more widely known and discussed. This is a neat summary of it:
We should remember the advice of Lord Renwick, a Foreign Office mandarin and Labour peer. He told young diplomats from good families that their background made them suckers for “the Wykehamist fallacy”. When they went abroad, they were in danger of believing that foreign potentates merely struck blood-curdling poses for effect. For all the bombast, they would think that, underneath, these must be civilised men with an ironic sensibility who might have been educated at Winchester. “They haven’t,” said Renwick. “Actually, they’re a bunch of thugs.”
This is a quote from a Nick Cohen article about UKIP. Nick hates UKIP so wants to cast them in a criminal light. Plenty of unpleasant people have been members of UKIP, but to call it a party of thugs is both inaccurate and counterproductive. Indeed, their widespread false characterisation as such almost certainly played a part in their being successful in their core and founding goal of getting the UK out of the EU.