The IQ levels of young people have been steadily falling for the past few decades, according to new research.

The decline is believed to have begun following the generation born in 1975, and indicates that the slow rise in intelligence observed over much of the 20th century has come to an end, says The Times.

Average IQs had risen by roughly three percentage points every decade since the Second World War, in a poorly understood trend known as the Flynn effect.

However, the new study, by Norwegian researchers, found that men’s IQs are measurably lower today than the scores of their fathers at the same age. Ole Rogeberg and Bernt Bratsberg, of the Ragnar Frisch Centre for Economic Research in Oslo, analysed the scores from a standardised IQ test taken by more than 730,000 Norwegian men who reported for national service between 1970 and 2009.

The results, published in the journal Proceedings of the National Academy of Sciences of the United States of America, or PNAS, show that those born in 1991 scored about five points lower than those born in 1975.

“This is the most convincing evidence yet of a reversal of the Flynn effect,” said Stuart Ritchie, a psychologist at the University of Edinburgh who was not involved in the research. “If you assume their model is correct, the results are impressive, and pretty worrying.”

The reasons for the Flynn effect and its apparent reversal are disputed. “Scientists have put the rise in IQ down to better teaching, nutrition, healthcare and even artificial lighting,” says The Times.

But “it is also possible that the nature of intelligence is changing in the digital age and cannot be captured with traditional IQ tests”, adds the newspaper.

An international study at the end of last year pointed towards the onset of technology as hindering the development of young people.

Researcher Michael Shayer, who co-authored the report, told Euronews that since 1995 a “large social force has been interfering with children’s development of thinking, getting larger each year”.

This “social force” includes the development of technology, such as game consoles and smartphones, “which have altered the way that children communicate with each other”, he explained.

“Take 14-year-olds in Britain. What 25% could do back in 1994, now only 5% can do,” Shayer added, citing maths and science tests.

Robin Morris, professor of neuropsychology at King’s College London, told The Times that IQ scores probably had hit a ceiling in the West, but there was not yet any reason to be unduly concerned.

“I think the reverse Flynn effect is real but would urge caution about generalising based on one sample,” he said. “Probably the tailing off is a general effect in high-income countries in which the contributor factors generally stabilise.”