This study aimed to determine the relations between fluid intelligence (Gf) and reading/mathematics and possible moderators. A meta-analysis of 680 studies involving 793 independent samples and more than 370,000 participants found that Gf was moderately related to reading, r = .38, 95% CI [.36, .39], and mathematics, r = .41, 95% CI [.39, 44]. Synthesis on the longitudinal correlations showed that Gf and reading/mathematics predicted each other in the development even after controlling for initial performance. Moderation analyses revealed the following findings: (a) Gf showed stronger relations to mathematics than to reading, (b) within reading or mathematics, Gf showed stronger relations to complex skills than to foundational skills, (c) the relations between Gf and reading/mathematics increased with age, and (d) family social economic status (SES) mostly affected the relations between Gf and reading/mathematics in the early development stage. These findings, taken together, are partially in line with the investment theory but are more in line with the intrinsic cognitive load theory, mutualism theory, and the gene-SES interaction hypothesis of cognition and learning. More importantly, these findings imply an integration model of these theories from an educational and developmental perspective: Children may rely on Gf to learn reading and mathematics early on, when high family SES can boost the effects of Gf on reading/mathematics performance. As children receive more formal schooling and gain more learning experiences, their reading and mathematics improvement may promote their Gf development. During development, the negative effects of low family SES on the relations between Gf and reading/mathematics may be offset by education/learning experiences. (PsycINFO Database Record (c) 2019 APA, all rights reserved).