A growing body of evidence indicates that research teams are increasing in size across almost every discipline. Several theories explain the rise in collaboration, including the accumulation of knowledge, declining communication costs, increasing capital intensity, shifting authorship norms, and increasing returns to research portfolio diversification. However, a paucity of empirical research focuses on disentangling these different mechanisms. This is a shortcoming of the literature because these different explanations yield distinct policy implications regarding, for example, the role of poverty traps in economic development, subsidies to higher education, and the composition of research evaluation committees. This study provides causal evidence of an increase in collaboration due to the accumulation of knowledge.