Teenagers in England are less intelligent than a generation ago, a study by a New Zealand researcher shows.
Professor James Flynn, of Otago University, found that IQ scores for the average 14-year-old had dropped by more than two points between 1980 and 2008. For those in the upper half of the intelligence scale, average IQ scores were six points down on 28 years ago.
Professor Flynn suggested that the falls could be down to lifestyle changes, including more time spent in front of the television or playing video games.
And a growing tendency in schools to "teach to the test" was affecting youngsters' ability to think laterally.
"While we have enriched the cognitive environment of children before their teenage years, the cognitive environment of the teenagers has not been enriched.
"Other studies have shown how pervasive teenage youth culture is, and what we see is parents' influence on IQ slowly diminishing with age."
The study contrasts with Professor Flynn's previous work, which suggested that intelligence has been consistently rising among all age groups in industrialised countries. The theory was dubbed the "Flynn effect".
He found that IQs increased among children aged between 5 and 10 over the 28-year-period, at the rate of up to half a point a year.
He said he believed that the gains were linked to changes to the home environment children experienced when they were young, with parents increasingly providing stimulating activities.
Professor Flynn's study, published in the Journal of Economics and Human Biology, was conducted using a test known as Raven's Progressive Matrices, where questions involve matching a series of patterns and sequences, so that even people with no education can take it.