MONDAY, Dec. 26 (HealthDay News) -- Children who have more schooling may see their IQ improve, Norwegian researchers have found.

Although time spent in school has been linked with IQ, earlier studies did not rule out the possibility that people with higher IQs might simply be likelier to get more education than others, the researchers noted.

Now, however, "there is good evidence to support the notion that schooling does make you 'smarter' in some general relevant way as measured by IQ tests," said study author Taryn Galloway, a researcher at Statistics Norway in Oslo.

Findings from the large-scale study appear in this week's online edition of the Proceedings of the National Academy of Sciences.

IQ, or intelligence quotient, is a widely accepted measure of intelligence. The IQ score comes from several combined, standardized tests.

In 1955, Norway began extending compulsory middle school education by two years. Galloway and her colleague Christian Brinch, from the department of economics at the University of Oslo, analyzed how this additional schooling might affect IQ.

Using data on men born between 1950 and 1958, the researchers looked at the level of schooling by age 30. They also looked at IQ scores of the men when they were 19.

"The size of the effect was quite large," she said. Comparing IQ scores before and after the education reform, the average increased by 0.6 points, which correlated with an increase in IQ of 3.7 points for an addition year of schooling, Galloway said.

"We are only able to study men, because we use data on IQ from the Norwegian military's draft assessment, which basically all men undergo around the age of 19. Women are not included in the draft," she explained.

Education has lasting effects on cognitive skills, such as those broadly measured by IQ tests, Galloway said.

"Cognitive skills are, in turn, related to a large range of social and economic outcomes. A large part of the relevance of the study derives from the fact that there has been some controversy related to the question of whether education has an independent effect on IQ or whether people with higher IQs simply choose, or are better able, to attain higher levels of education," Galloway said.

By looking at a reform which increased mandatory schooling and prevented people from dropping out of school after the 7th grade, it is fairly certain that the effects seen are an effect of schooling on IQ, not vice versa, she explained.

"One subtle point of our findings is that we use IQ measures at roughly age 19, which is three to four years after the additional education generally was received. Thus, we are not simply picking up a short-lived effect that peters out shortly after people leave school," Galloway said.

The findings suggest that education as late as the middle teenage years may have a sizeable effect on IQ, but do not challenge the well-documented importance of early childhood experiences on cognitive development, according to the authors.

Robert Sternberg, a professor of psychology and provost at Oklahoma State University in Stillwater, said that "these results -- that schooling has a substantial effect on IQ -- replicate those of other, perhaps not quite as well-controlled, studies."

"I am aware of no serious studies that show the opposite result," he added.

He said the results are also consistent with the huge literature on the so-called Flynn effect showing that IQs are modifiable across as well as within generations and have been rising since the beginning of the 20th century.

"The results of this study are problematical for the chorus of psychologists and educators still locked in century-old thinking that IQ is genetic, stable and non-modifiable," Sternberg said. "As, for these individuals, the belief in the stability of IQ is more a matter of religious faith than of scientific inference, I doubt they will be persuaded."

More information

For more about IQ, visit the U.S. National Library of Medicine.