Increasing geographical and cultural diversity in research participation has been a key priority for psychological researchers. In this article, we track changes in participant diversity in developmental science over the past decade. These analyses reveal surprisingly modest shifts in global diversity of research participants over time, calling into question the generalizability of our empirical foundation. We provide examples from the study of early child development of the significant epistemic and ethical costs of a lack of geographical and cultural diversity to demonstrate why greater diversification is essential to a generalizable science of human development. We also discuss strategies for diversification that could be implemented throughout the research ecosystem in the service of a culturally anchored, generalizable, and replicable science. (PsycInfo Database Record (c) 2024 APA, all rights reserved).