Does spanking kids make them more aggressive?

A recent study in Child Development and reported in the San Francisco Chronicle says spanking kids at age 1 makes them more aggressive at age 2 than kids who haven't been spanked, but spanking them at 2 doesn't make them more aggressive at 3. This makes me think the first result might be some sort of artifact. Why would spanking make kids more aggressive at 1 but not at 2?

It also said the kids spanked at 1 did worse on cognitive tests at 3 than kids who weren't spanked. This makes me wonder if the causation goes the other way around: People who have kids with lower cognitive abilities are more likely to spank their kids.