A study of Norwegian men has revealed that an increase in the number of years spent in school may boost intelligence.
The research suggested that an extra year in the classroom could increase IQ by nearly four points.
But they do not know if this applies to all children, or just those in the study.
Researchers from Statistics Norway, which publishes official government data, and the University of Oslo took advantage of a natural experiment in the Norwegian education system and its effect on 107,223 pupils.
Between 1955 and 1972 regional governments in Norway increased compulsory schooling from seven to nine years. It meant pupils left school at 16 instead of 14.
The effect of this forced increase in schooling was measured at the age of 19, when the military gave all men eligible for drafting an IQ test.
"An unusually large increase in both average education and average IQ is apparent at the same time as the reform was introduced," BBC quoted the researchers as saying.
However, determining whether spending more time in school actually improves IQ is more difficult, as it is possible that children with a naturally higher IQ are those who choose to spend more time in the education system.
The statisticians involved in the study also cautioned against drawing too many conclusions, as they admit that the effect may only apply to Norwegian society or its education system at the time.
The study was published in Proceedings of the National Academy of Sciences.