General intelligence, broadly defined as the ability to perform well on many and even seemingly unrelated cognitive tasks (e.g. maths and reading), is one of the most documented and empirically supported findings in psychology. Moreover, general intelligence has consistently been associated with important life outcomes such as educational achievement and occupational success. For instance, people with greater general intelligence (often measured by IQ tests) tend to get better grades in school and earn more income over their lifetime. However, despite the robustness of and cultural attention given to intelligence research, little is known about how it develops, especially in childhood and adolescence. In this Perspective, I introduce a theory of cognitive development known as mutualism, which derives its name and theoretical origins from the ecological interaction between two species in which each receives a net benefit (e.g. bees extracting nectar for nutrients from flowers in exchange for pollination). According to mutualism theory, general intelligence emerges from positive interactions between cognitive abilities such as reading and maths so that, over time, they become more related to one another. This would explain why people who are good at one task also tend to perform well on others. Lastly, I discuss possible applications of mutualism to education policy, particularly focussing on ways to improve the performance of students who struggle to learn in school.