Little attention has been paid to the role that low levels of cognitive development (or IQ) play among both left-behind children (LBCs) and children living with parents (CLPs) in the context of poor educational attainment in rural China. In this paper, we examine how general cognitive abilities contribute to the academic achievement gains of both LBCs and CLPs in poor areas of rural China. We measure the general cognitive ability of the 4,780 sample students using the Raven's Standard Progressive Matrices (Raven IQ) and assess academic achievement using a curriculum-based mathematics exam. We find that IQ and left-behind status predict achievement gains for the average student. Among low-IQ students, however, left-behind status does not correlate with a change in achievement, suggesting that the migration of parents does not immediately/automatically translate into a loss of academic achievement for students with delays in their general cognitive ability.