Early in my interests in SEO I learned something about the "popularity component" that is built into most search engines. Within the "popularity component" was, of course, the number and quality of links pointing to your site but in addition, there was the idea that CTR (click-through rates) was a metric used by the search engines to rank pages naturally in the free results.
A thread at HighRankings brings this memory back to me. Since then, I have not considered that Google or other popular engines use CTR to rank sites in the free results. Pretty much all the well known search engine optimization figures say that no major search engines uses such a factor in ranking. However, some speculate that some of the smaller engines use it.
Interesting how we remember things like this, if I am correct, the book that had this was Shari Thurow's Search Engine Visibility.