The intellect of even the brainiest 14-year-olds has deteriorated dramatically over the decades despite an increase in the number of pupils achieving top grades in exams.
Their cognitive abilities are level with those of 12-year-olds in 1976, the study found.
Researchers at King's College London compared the mental agility of 800 bright 13 and 14-year-olds with similar tests carried out some three decades ago.
The tests - designed to assess grasp of abstract scientific concepts such as volume, density, quantity and weight - found far fewer youngsters hit top scores than in 1976.
In one test, average achievement remained roughly the same as in 1976.
But only just over one in ten pupils were able to demonstrate a "higher level of thinking" compared to one in four in 1976.
In a test to measure mathematical abilities, only one in 20 of today's teenagers were top scorers, down from one in five in 1976.
Professor Michael Shayer, who led the study, said the brainpower slump may be down to over-testing in schools.
He said: "The moment you introduce targets, people will find the most economical strategies to achieve them.
"In the case of education, I'm sure this has had an effect on driving schools away from developing higher levels of understanding."
In previous research, Professor Shayer concluded that the cognitive abilities of 11-year-olds were up to three years behind where they were in 1975.