The researchers - Peera Wongupparaj, Veena Kumari and Robin Morris at Kings College London - did not themselves ask anyone to sit an IQ test, but they analysed data from 405 previous studies. Altogether, they harvested IQ test data from more than 200,000 participants, captured over 64 years and from 48 countries.

Focusing on one part of the IQ test, the Raven's Progressive Matrices, they found that on average intelligence has risen the equivalent of 20 IQ points since 1950. IQ tests are designed to ensure that the average result is always 100, so this is a significant jump. [....]

The new research is further confirmation of a trend that scientists have been aware of for some time. In 1982, James Flynn, a philosopher and psychologist based at the University of Otago in New Zealand, was looking through old American test manuals for IQ tests. He noticed that when tests were revised every 25 years or so, the test-setters would get a panel to sit both the old test and the new one.

"And I noticed in all the test manuals, in every instance, those who took the old test got a higher score than they did on the new test," says Flynn. In other words, the tests were becoming harder.

This became known as the Flynn Effect, though Flynn stresses he was not the first to notice the pattern, and did not come up with the name.

But if the tests were getting harder, and the average score was steady at 100, people must have been getting better at them. It would seem they were getting more intelligent.

If Americans today took the tests from a century ago, Flynn says, they would have an extraordinarily high average IQ of 130. And if the Americans of 100 years ago took today's tests, they would have an average IQ of 70 - the recognised cut-off for people with intellectual disabilities. To put it another way, IQ has been rising at roughly three points per decade.