[Varying a lot of experimental parameters is] much easier to do in physics than psychology– physics apparatus is complicated and expensive, but once you have it, atoms are cheap and you can run your experiment over and over and over again. Human subjects, on the other hand, are a giant pain in the ass– not only do you need to do paperwork to get permission to work with them, but they’re hard to find, and many of them expect to be compensated for their time. And it’s hard to get them to come in to the lab at four in the morning so you can keep your experiment running around the clock.

From On Black Magic in Physics