Background: Multiple perspectives on cognitive development, including Cattell's Investment Theory (1963, 1987), suggest that fluid reasoning (Gf) is foundational to the development of academic knowledge and skills. Yet little is known of the role that Gf may play in reading achievement, particularly in comparison with a larger literature demonstrating an influential role of crystallised intelligence (Gc) in word recognition. Given the novelty of word recognition for early readers, it is likely that Gf plays an influential role in literacy acquisition. Method: Using regression-based mediation analysis, the present study examined performance of preschool children (n = 154) and college-aged adults (n = 114) on measures of Gf, Gc, phonemic decoding and word recognition. Results: Results show that Gf has a significant direct effect on word recognition among preschoolers but not among adults. Moreover, among preschoolers, Gf exerted an effect indirectly on word recognition through phonemic decoding (i.e., Gf ? phonemic decoding ?word recognition), as well as through a serial and joint mechanism of Gc and phonemic decoding (i.e., Gf ?Gc ?phonemic decoding ?word recognition). However, among adults, Gf exerted an effect onto word recognition only through this joint and serial mechanism and not singularly through phonemic decoding. Conclusions: Present findings link theories of cognitive development with models of reading development, suggesting that while Gf may both directly and indirectly invest in word recognition during early literacy acquisition, this investment may "shift" in adulthood such that Gf exerts only indirect effects through crystallised knowledge and skills (i.e., Gc and phonemic decoding).