IQ levels have been declining since the era of Harry Dunne and Lloyd Christmas

The IQ scores of young people have begun to fall after rising steadily since the Second World War, according to the first authoritative study of the phenomenon.

The decline, which is equivalent to at least seven points per generation, is thought to have started with the cohort born in 1975, who reached adulthood in the early Nineties.

Scientists say that the deterioration could be down to changes in the way maths and languages are taught, or to a shift from reading books to spending time on television and computers.

Yet it is also possible that the nature of intelligence is changing in the digital age and cannot be captured with traditional IQ tests. The turning point marks the end of a well-known but poorly understood