This study addresses the investment hypothesis of fluid on crystallised abilities onto academic achievement (Gf [right arrow] Gc [right arrow] "Achievement"), which might hold to a greater extent at earlier than at latter educational stages. We compared this prediction with two independent groups of secondary (n = 192, 113 females) and university students (n = 210, 178 females). Structural equation models were used to evaluate the association between Gf-Gc and general intelligence (g) with academic achievement. Moreover, direct and mediation models (Gf [right arrow] Gc [right arrow] "Achievement") represented the data better than a g-factor model in both groups of students. Direct and indirect effects of Gf-Gc were alike across both groups of students. A larger amount of variability in achievement, however, emerged for secondary students compared with university students, suggesting a more robust support for the investment hypothesis at the earlier educational stage.