Those who know him say that Boris Johnson is an intelligent man. If he took an IQ test tomorrow he would no doubt score in the top 5 per cent, especially if the questions were set in Latin. He is by all accounts smart, bright, clever and cunning – the latter being especially useful for a politician and serial philanderer.
Johnson's wit and mental alacrity are indisputable. He is widely read, well-educated (Eton and Oxford) and a skilled raconteur who can tell a good joke. But there are other ways of describing intelligence that may not seem quite so apt for the accident-prone London Mayor.
How many people, for instance, would happily describe him as wise or sensible? And surely he would hardly score above average for emotional intelligence or the ability to empathise with other human beings, especially those less fortunate and privileged than himself.
Nick Clegg said yesterday that Johnson displayed "careless elitism" when advocating that more should be done to help the intelligent wealth-creators of society, and that Johnson was being "fairly unpleasant" by talking about people as if they were a breed of dog.
What so offended Clegg was Johnson's description of the innate intellectual inequality of humans, especially those "of our species" with the lowest IQ scores. When he delivered the annual Margaret Thatcher lecture on Wednesday evening, Johnson said that humans were far from being equal in "raw ability".
"Whatever you may think of the value of IQ tests, it is surely relevant to a conversation about equality that as many as 16 per cent of our species have an IQ below 85, while about 2 per cent… have an IQ above 130," Johnson told his audience.
But any discussion of IQ tests should consider their value. What are they actually measuring, how well do they do it and what does the end result really mean?
Whenever politicians talk about intelligence and IQ, they risk being drawn into a quagmire of controversy going back over half a century. One of the problems is that experts themselves cannot agree on what is meant by intelligence, how to measure it and what that IQ metric actually stands for.
Leaving aside the word itself, and all its different connotations of cognitive ability, a universal scientific definition of intelligence does not seem to exist. Indeed, when two dozen prominent psychological theorists were asked to define intelligence in the 1980s, they came up with two dozen somewhat different definitions.
Intelligence can also be measured in different ways by a variety of psychometric tests. Some are better at testing linguistic ability, others are better at judging numeracy or spatial ability. And all tests try with varying degrees of success to overcome cultural biases – a test set in Latin, for instance, would hardly be fair on someone who didn't go to Eton.
By convention, intelligence tests are scored on a scale in which the mean is set at 100 and about 95 per cent of the population will have an IQ score which falls within two standard deviations of the mean, meaning they will fall between 70 and 130.
The bell shape of the resulting curve is probably one of the most famous graphs in psychology. Indeed, the 1994 book The Bell Curve was named after it. It was this book more than any other over the past 20 years that epitomised the bitter rancour over intelligence and IQ.
The authors, Richard Herrnstein and Charles Murray, argued that IQ was largely determined by a person's genes and upbringing, and was a better predictor of financial income and career success than the socioeconomic status of the individual's parents. Boris Johnson was simply expressing the same kind of deterministic, right-wing ideology.
Other academics have shown, however, that IQ is not necessarily fixed in childhood, nor is it accepted that IQ has a strong genetic heritability. One of the strongest correlations of financial and economic success is still the size of your parents' bank account – and whether they could afford to send you to a posh school with impeccable social connections.