You might think it's obvious that one person is smarter than another.
But there are few more controversial areas of science than the study of intelligence and, in reality, there's not even agreement among researchers about what this word actually means.
Unlike weight and height, which are unambiguous, there is no absolute measure of intelligence, just as there are no absolute measures of honesty or physical fitness.
Nonetheless, over the decades, legions of scientists have devised tests that can show that one person is smarter than another just as surely as Olympic events can shed light on how much you can lift or how far you can jump.
Now my team at the UK Medical Research Council's Cognition and Brain Sciences Unit in Cambridge has come up with the ultimate test of intelligence.
Like many researchers before us, we began by looking for the smallest number of tests that could cover the broadest range of cognitive skills that are believed to contribute to intelligence, from memory to planning.
