Does getting spanked make kids stupid?

On the CBS Early Show just now, they were talking about this study.

OK, everyone take a deep breath and repeat after me: Correlation does not imply causation

One possible hypothesis is that spanking a child somehow lowers that child's IQ. That's the idea that the news people seem to be taking away from this study. I haven't read the actual study, that may even be what the authors think. But it's never safe to assume that the mass media are accurately reporting the results of a scientific study.

Please take a moment and consider this alternate hypothesis: IQ is largely hereditary, and less intelligent parents are more likely to resort to corporal punishment.