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Hernán, M. A., Clayton, D., & Keiding, N. (2011). The Simpson’s paradox unraveled. International Journal of Epidemiology, 40(3), 780–785. https://doi.org/10.1093/ije/dyr041

The one thing I'd note is the personality confound -- the people who get inspired by profs, seek out extra work, involve in extra-curriculars, etc. may just be people who approach life with a better attitude period, or may be from a social class that allowed them to do these things.