A newish patent application from Yahoo named Identifying excessively reciprocal links among web entities was filed on January 8, 2009. The patent inventors include Priyank Garg and Tim Converse of Yahoo, both well-known search engineers at Yahoo. Of course, we have an excellent and thorough explanation of that patent from Bill Slawski. We also have a write up from David Harry on the patent app.
Here is the abstract:
A method for identifying reciprocal links is provided. At a particular host, the set of hosts which link to the particular host and the set of hosts to which the particular host links are determined. The intersection and union of the two sets of hosts are also determined, and the sizes of the intersection and union are calculated. The concentration of reciprocal links at the particular host is calculated based on the sizes of the intersection and union. A ratio of the intersection size to the union size is used to determine the concentration of reciprocal links. The particular host's rank in a list of ranked search results may be changed as a result of identification of a high concentration of reciprocal links.
So I am not going to repeat the summaries of this patent app, for that, use the links above. There is a lot of discussion around it.
Threads at Sphinn, WebmasterWorld are starting to get their minds 'wet' with debate over the patent application. Of course, it is always important to remember that a patent or patent app does not mean a search engine is actually using the methods in the current algorithms, nor does it mean they are using it exactly like it is written in the patent.
Martinibuster adds a nice point in the Sphinn thread saying:
What's more interesting is the execution of how links are rewarded. Big difference between Yahoo and Google, with Google tending to favor the user experience and Yahoo seeming to focus on keeping spam out to the detriment of the search results.