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 the present paper, the G8 reform was utilized as a natural quasi-experiment to examine whether the duration of school attendance would enhance intelligence test scores even when students with different numbers of years of schooling had completed the same curricular contents within the same amount of instruction. Two studies were conducted. In Study A, the performances of n = 81 G8 10th graders (Mage = 15.16 years, SD = 0.37) and n = 80 G9 11th graders (Mage = 16.39 years, SD = 0.49) on the Berlin Intelligence Structure Test were compared. In Study B, the cognitive abilities of n = 244 G8 10th graders (Mage = 15.23 years, SD = 0.42) and n = 204 G9 11th graders (Mage = 16.33 years, SD = 0.47) were measured with the Intelligence-Structure-Test 2000 R. The G9 students outperformed the G8 students on nearly all cognitive ability tests despite completing equal curricula. Thus, the impact of schooling on intelligence test scores seems to be primarily due to the fostering of intelligence-related abilities that are independent of formal curricula.