A major study carried out in Norway suggests IQ scores among men there have been falling since the mid-1970s.

The study involved more than 700,000 men born to Norwegian couples between 1962 and 1991. The research was carried out by Oslo’s Ragnar Frisch Center for Economic Research. Results were published last week in the U.S.-based scientific journal Proceedings of the National Academy of Sciences.

The men were given tests around age 18 to measure IQ -- or intelligence quotient -- as part of required military service in Norway. IQ tests are designed to measure intelligence based on areas such as vocabulary knowledge, verbal and non-verbal reasoning skills, and working memory.

In the Norwegian study, results showed the average IQ score increased about three percent for men born between 1962 and 1975 -- from 99.5 to 102.3.

But the scores began dropping for men born after 1975. By 1989, the average IQ score had returned to 99.4.

The study is related to something known as the Flynn effect. This is the idea that if the same IQ tests are given to people born at different points in time, the scores will generally rise.

Ole Rogeberg is with the Frisch Center and was a lead researcher on the study. He told VOA the Flynn effect showed a clear increase in IQ scores during the 20th century throughout the Western world.

Researchers found the IQ increases during this time period happened too fast to be related to genetics. They said the higher scores were instead caused by environmental factors. Among possible factors were better teaching and learning methods, improvements in test-taking skills and healthier eating.

Rogeberg said that in the new study, he wanted to examine possible causes for the steady drop in IQ scores. To rule out genetics, he attempted to find similarities between brothers.

“Because on average, the genes that the first born and the second born and the third born get, they will be relatively similar. There’s no reason to expect that one sibling gets better genes just because he’s the first or second child.”

He concluded that external factors probably influenced the scores more than genetics. However, he added that the study did not provide any information about specific kinds of environmental causes for the lower IQ scores. But he said researchers are considering a number of theories.

“It may be due to changes in the educational system or that it reflects a change in the media environment -- if people used to read books more, but now they're watching television more or going on the internet more.”

Rogeberg looks forward to results of additional studies that can support his findings. He says more research is needed to help explain all the reasons behind changes in IQ scores.

“It's difficult to compare the results across time, and whether it actually reflects a change in some underlying intelligence or whether it reflects differences in how we have been trained to use our intelligence to solve different kinds of tasks.”

In any case, Rogeberg says he believes the reasoning and critical thinking skills measured by intelligence tests will always be important. He notes that humans will still need to process information, even as technology and artificial intelligence become a bigger part of people’s lives.

“You have to be able to reason your way through the world as you live it,” he said. “And I don't think these skills are going to lose their relevance. But the way they are relevant might change.”