Social loafing, also known as "lurking", greatly affects the development and growth of online communities. The term "social loafing" refers to the tendency for individuals to expend less effort when working collectively than when working individually. This phenomenon is much like people's tendency to be part of a group project, but rely heavily on just a few individuals to complete the work. Generally, social loafers regularly follow the discussions and content of online communities, but choose not to expand on posts or add to the knowledge of the community. Additionally, participation in online communities is usually voluntary; therefore there is no guarantee that community members will contribute to the knowledge of the website, discussion forum, bulletin board, or other form of online engagement. Lurkers are reported to constitute over 90 percent of several online groups.