[ghaff]

There are a whole variety of readability tests [1] and many of them consider some of the variables such as word length that he did. Mind you, readability is not the same thing as intelligence. Especially in pop music, I wouldn't expect to see a lot of "SAT words." I'm not sure how you can really measure intelligence of song lyrics but, that said, some sort of complexity metric is probably a reasonable proxy to consider.

[1] http://www.ideosity.com/ourblog/post/ideosphere-blog/2010/01...