From pages 97-98 of Opening Up by James Pennebaker:
This is a pretty good summary of GTD from an experimental psychologist.
Many times, when we write something down, we don't have to think about it any longer. I've noticed this in myself when I'm preparing to go on a vacation. There's packing, stopping The mail and newspapers, getting the car checked, and on and on. In the middle of meetings or talking to someone on the phone, overlooked chores come to mind: "Oh, I can't forget to pack the fishing rod," or "Get someone to water the plants." As much as I try to avoid it, I usually break down and start making lengthy lists of last-minute tasks to perform. Before list making, I actively juggle the tasks in my mind. Once I start the lists, however, my mind becomes freer and I feel less distressed. I have, in essence, transferred my mental notes from my head onto a piece of paper.
This is a pretty good summary of GTD from an experimental psychologist.