The research examines possible differences in the mutual reinforcement (i.e. multiplicative) effect between intrinsic and instrumental motivation on academic performance across different cultures. Eight representative countries and economies from two large‐scale databases—the Trends in International Mathematics and Science Study (TIMSS) 2011 and the Programme for International Student Assessment (PISA) 2012—were used, respectively. Results showed that among the Confucian economies, instrumental motivation was more helpful in improving the mathematics performance of students with low intrinsic motivation than for those with high intrinsic motivation. This was shown by the multiplicative effect between intrinsic and instrumental motivation. Despite the difference, students in both Confucian and Western cultures with high intrinsic motivation had better mathematics performance than students with low intrinsic motivation.