The study was conducted by James Flynn, a leading researcher of intelligence quotient (IQ) measurement, writes Norwegian news agency NTB.
IQ in the Scandinavian countries had been on the up for decades, but the trend has now been reversed, according to a report published by journal Intelligence.
Measures taken as part of the study have shown that, in Norway, fewer people are reaching higher levels of mathematical and linguistic ability.
The trend is similarly prominent in both Denmark and Norway and even more so in Finland.
“This signals without doubt a depletion of people with high cognitive abilities,” Flynn said according to NTB's report.
The IQ of Norwegians had been increasing since 1950 until a turning point around 1995, writes the agency.
An annual reduction of IQ by 0.21 percent on average has been measured in Norwegians since that year, equivalent to a 6.5 percent decrease between generations.
According to New Zealand researcher Flynn, intelligence levels had been increasing steadily in Scandinavia due to more abstract and rational thinking in schools and at workplaces.
But improvement of the quality of schooling has now stopped, he concluded.
“We have seen that work in schools is less demanding in many countries and that students spend less time studying and doing homework,” Flynn said to Swedish magazine Forskning & Framsteg, according to NTB's report.