What happens if the same curriculum is taught in five instead of six years? A quasi-experimental investigation of the effect of schooling on intelligence

Several studies have shown that each year of schooling has a significant positive impact on IQ. However, there is still a debate about how to explain this effect. With the German G8 school reform, the duration of school attendance in the highest track of Germany’s tracked school system (Gymnasium) was shortened from 9 years (G9) to 8 (G8) while simultaneously both the curricular contents and amount of instruction should be preserved in full. In...