Behavior-based associations, such as item-to-item or query-to-item associations, are extrapolated to other items to create new associations. The items to which the associations are extrapolated may be “behavior deficient” items, or items for which the quantity of collected user activity data is insufficient to create meaningful or reliable behavior-based associations. The behavior-based associations are extrapolated based on content-based associations, or another type of “substitutability” association, between items. The items can be any type of item (e.g., products, web sites, documents, etc.) for which user behaviors (e.g., purchases, accesses, downloads, etc.) can be monitored and analyzed to detect behavior-based associations, and for which item content or other available information can be used to assess item substitutability.