Intelligence testing can be used to determine mental development. I think this is important to the history of psychology because we started trying to understand mental development while looking at intelligence. There is a slight correlation to intelligence testing and the influence for mental development.

Orben, A., Tomova, L., & Blakemore, S.-J. (2020). The effects of social deprivation on adolescent social development and mental health [Preprint]. PsyArXiv. https://doi.org/10.31234/osf.io/7afmd