It supposedly correlates because low IQ people are more likley to be moderate, or at lease less likley to identify with labels such as conservative or liberal.
There was one study that I can't seem to find on a quick google, but using WORDSUM from the GSS as a proxy for IQ, Razib Khan from Gene expression gets basically the same result.
Here's the news article on this: http://www.yourmorals.org/blog/2011/11/how-to-use-groups-at-yourmorals-org/
And here's the group that the LW community just created: http://www.yourmorals.org/setgraphgroup.php?grp=623d5410f705f6a1f92c83565a3cfffc
I think it will be very interesting to see what we can all get on this.