Theists one-box because they tend to be more willing to accept backwards causation at face value (just guessing, but that's what I'd expect to find). Undergraduates surprise me; I would have expected that more of them would two-box, but I may be overestimating the exposure most of them have to formal decision theory. LW readers one-box because we believe rationalists should win, and because of EYs twelfth virtue of rationality; with every action, aim to cut, not merely to be "rational" or "Bayesian" or any other label. Most arguments for two-boxing refuse to allow the stated accuracy rate or complain bitterly that they're being punished for being "rational". In reality, they are failing to understand or accept that the correct causal diagram is your past state of mind causing both your current decision and the box in which the money is contained. This can be due to any number of factors, including most often I would suspect confusion about free will and the nature of a deterministic universe.