Spending more time at school may increase intelligence, according to a study of Norwegian men.
The research, in Proceedings of the National Academy of Sciences, suggested that an extra year in the classroom could boost IQ by nearly four points.
The authors said that extra schooling had a "sizeable effect" on IQ in early adulthood.
But they do not know if this applies to all children, or just those in this study.
A link between a higher IQ and more education has already been established, the authors say.
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.
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.
The researchers reported that: "An unusually large increase in both average education and average IQ is apparent at the same time as the reform was introduced."
They said one additional year in school raised IQ by 3.7 points.
They added: "By exploiting the increase in schooling induced by a comprehensive compulsory schooling reform, this study is able to uncover a statistically significant and sizeable effect of middle school education on IQ scores in early adulthood for Norwegian men."
The statisticians caution against drawing too many conclusions, as they admit that the effect may only apply to Norwegian society or its education system at the time.
However, they argue that it may be possible to improve intelligence in adolescence.