If you want to be shunned in polite society, sometimes all you have to do is state the obvious. Last week Boris Johnson, London's flamboyant bad-boy mayor, dared to state the obvious. In the course of a long speech about equality and meritocracy, he said that inequality is inevitable, because intelligent people are more likely to succeed than less intelligent ones.

And then the fur flew.

The Guardian (voice of the righteous left) accused him of mocking stupid people, and wondered if he'd blown his chances at the Tory leadership. Nick Clegg, the deputy prime minister, attacked him for "unpleasant, careless elitism." Leading Tories scurried away in horror. Experts were consulted to prove that Boris knew nothing about IQ, a concept so scary and embarrassing that it is unmentionable in public life.

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Here's some of what he actually said. "Whatever you may think of the value of IQ tests, it is surely relevant to a conversation about equality that as many of 16 per cent of our species have an IQ below 85, while about 2 per cent have an IQ above 130. The harder you shake the pack, the easier it will be for some cornflakes to get to the top." And: "The income gap between the top cornflakes and the bottom cornflakes is getting wider than ever."

In Mr. Johnson's defence, what he said about IQ was not a value judgment about people. It was merely an accurate description of how the Bell curve works. Only in Lake Wobegon are all the children above average. He was right about the income gap, too.

The correlation between intelligence and success is far from perfect, but it's also very far from random. Some people who rise high in public life are stunningly dim (insert obvious example here). But in general, Supreme Court judges are smarter than the rest of us, and newspaper readers are smarter than the average prison inmate (unless his name is Conrad Black). And in spite of the strenuous efforts of our cognitive elites to stress nurture over nature, we're more or less born with the intelligence we've got.

Of course, even smart people can be thick as a brick. Take Larry Summers, the former president of Harvard, who lost his chance to run the Federal Reserve because he'd once mused about why the world's top scientists include so few women. That was beyond stupid. (It was also Mr. Summers who said that Harvard's "A" students wind up as professors, while the "C" students wind up as donors.)

It's not hard to see why IQ is considered too toxic to discuss in public. The idea that there are important differences in intelligence offends our most cherished liberal values. It sounds shockingly deterministic. It means the playing field isn't level, no matter what we do. It means that some people are born with inherent advantages, and that inequality (as Mr. Johnson said) is unavoidable no matter what we do.

For many years it was fashionable to proclaim that IQ did not not exist, or that it didn't matter, or that it couldn't be measured anyway. So imagine my surprise to hear a CBC Radio host discussing the genius of Mozart the other day. She came right out and said he had an IQ of 165 – the same as Darwin. It was the most thrilling and transgressive thing I've heard on the CBC in years.

Yet the idea that the world will increasingly be run by smart people (instead of, say, ruthless thugs) is not that reassuring. The meritocratic elites have shown that they're as capable of making a total mess of things as anybody else. William F. Buckley once said he'd rather be governed by the first two thousand names in the Boston telephone directory than by the faculty of Harvard. His point was that smart people aren't nearly as smart as they think they are. Instead of being smug, they should try humility.