From Jonathan Allen:

Re: Lower IQ’s measured in spanked children (Sept. 24): The horizontal axis of the graph is labeled “number of times spanked.” Is this the number of times per day, per month, ever? Is it also possible that intelligence is to some extent heritable?

That is, smart parents beget smart kids and stupid parents beget stupid kids. Stupid parents also spank more, but could this fact be incidental to, rather than causal?