Middle-class teenagers are less intelligent than a generation ago due to the dumbing down of youth culture and school tests, a new study suggests.
IQ tests show that scores for the average 14-year-old have dropped by more than two points between 1980 and 2008.
For those in the upper half of the intelligence scale--a group typically dominated by the children of middle-class families--average IQ scores were six points down on 28 years ago.
Leisure time is increasingly taken up with playing computer games and watching TV instead of reading and holding conversations.
Education experts said a growing tendency in schools to "teach to the test" was affecting youngsters' ability to think laterally.
For the study, Professor James Flynn, of the University of Otago in New Zealand, analysed UK children's scores in IQ tests over time.
He found that IQs increased among children aged between five and 10 over the 28-year-period, at the rate of up to half a point a year.
He believes these gains are linked to changes to the home environment children experience when they are young, with parents increasingly providing stimulating activities.
But Professor Flynn also found that teenagers' scores had dipped slightly over the past generation.
It is the first time IQ scores have fallen for any age group during the past century, his research suggests.
"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."
"What we know is that youth culture is more visually orientated around computer games than they are in terms of reading and holding conversations."
He said previous studies have tended to show IQ increasing as teenagers move into adulthood, entering university or starting work.
Previous studies have claimed that using text messages and email can temporarily reduce IQ by causing concentration to drop, while smoking marijuana has also been linked with a decline in IQ.