Cool, so what kinds of people typically leave and what kinds of people typically stay?
"Oh... I was going for promo."
Self selection would be one example. To be fair, when I once had a beer with someone who worked in that area before coming to academia she basically said you could work out how many intelligence officers a country had, and realise that given the amount of time effort and money it takes to properly surveil someone they were limited to a small multiple of that number of people to spy on "properly". I felt reassured by that. I can also see it being a justification for RoboCop.