Young people's IQ scores have started to deteriorate after climbing steadily since World War Two, a new study has found.
The fall, which equates to about seven points per generation, is believed to have begun with those born in 1975, according to the first authoritative study of the phenomenon.
The drop in scores marks the end of a trend – known as the Flynn effect – which has seen average IQs rise for the past 60 to 70 years by roughly three points a decade.
Scientists have described the results as 'impressive' but 'pretty worrying', according to the Times.
The decline is to do with a difference in the way languages and maths are taught in schools, scientists have suggested.
However, it could also be down to people spending more time on technological devices instead of reading books.
Stuart Ritchie, a psychologist at the University of Edinburgh who did not take part in the research, told the newspaper: 'This is the most convincing evidence yet of a reversal of the Flynn Effect.
'If you assume their model is correct, the results are impressive, and pretty worrying.'
However IQ scores might have fallen since the turn of the millennium, according to previous studies.
Two British studies suggested that the fall was between 2.5 and 4.3 points every ten years.
But due to limited research, their results were not widely accepted.
In the latest study Ole Rogeburg and Bernt Bratsberg, of the Ragnar Frisch Centre for Economic Research in Oslo, found that Norwegian men's IQs are lower than the scores of their fathers when they were the same age.
The pair analysed the scores from a standard IQ test of over 730,000 men – who reported for national service between 1970 and 2009.
The findings come after scientists revealed in December 2017 that regularly eating fish improves children's intelligence as well as helping them to sleep better.
They found that nine to 11-year-olds who eat it at least once a week scored almost 5 points higher in IQ tests to those who 'seldom' do.
In this study by US researchers, more than 500 children were asked about how often they had consumed fish in the past month. They then took part in an IQ test which also considered such as verbal and written communication skills.
After taking into consideration factors such as their parental education, occupation and marital status, it found children who eat fish least once a week score 4.8 points higher than those who never do. Even those whose meals sometimes include fish scored 3.3 points higher.