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&M did their PhD and postdoc.