but what would you have them do? Hit small children and then track them when they're older? It's not perfect, far from it, but the researchers only have so much to work with. At some point in time, they are relying on personal accounts. Psychology experiments can't exactly fit the bill of most hands-on experiments because of all sorts of moral and time issues. Given what they have to work with, I think it was sufficiently designed. Certainly better than that stupid g-spot study

EDIT: Oh, and also I note two things. One, this is not the body of the research, it is The Guardian reporting on it. So it's hardly like it does the piece justice. Two, you'd be amazed what one remembers. One would at least be able to remember generalizations of frequency and intensity. I could probably count on one hand the number of times I was physically punished and with a big enough pool, that's enough for a statistical comparison.