1. A computer-implemented method of detecting associations between items represented in an electronic catalog, the method comprising: maintaining item selection histories of each of a plurality of users of the electronic catalog, said item selection histories identifying particular items selected by particular users; detecting a behavior-based association between two items represented in the electronic catalog, item A and item B, at least in part by programmatically analyzing said item selection histories; detecting a substitutability association between item A and an additional item, item C, represented in the electronic catalog; and programmatically using the detected behavior-based association between items A and B, in combination with the detected substitutability association between item A and item C, to create, in computer storage, an association between item B and item C.