Do people need to have their PhDs at Harvard to get tenure at MIT (and vice versa) in STEM fields? My friends and I were just discussing about this question: do people need to have their PhDs at Harvard to get tenure at MIT (and vice versa) in STEM fields?  I know theoretically, the answer is supposed to be "No", and "It depends on many other factors". But I just skimmed through MIT's Biology and Harvard's Biophysics department; it appears that those two giants only want to marry each other. 

I wonder if there are are any detailed statistics of where faculty from H&amp;M did their PhD and postdoc.
I have never sat on a tenure case at either Harvard or MIT, so my answer is purely conjecture. While it is possible the first thing the tenure committee does is look at where the PhD is from and throws out everyone not from Harvard/MIT, that seems very unlikely. Tenure decisions are about teaching, service, and research. At the point of tenure decisions, the school your PhD is from is essentially meaningless although there may be some correlation between research productivity and where your PhD is from any longer term causative effects are probably pretty limited.

Where the PhD is from plays some role on initial hiring at all universities and if you don't have a TT position it is much harder to get tenure. Additionally, where you work probably has some causative effect on research productivity giving Harvard and MIT PhDs additional advantages (or at least preventing disadvantages). Therefore it is not surprising that lots of tenured faculty have PhDs from Harvard and MIT.