I wrote a column in the Washington Post some time ago exploring the work of Eli Berman and David Laitin, who argued that terrorist groups function much in the manner of exclusive country clubs. (OK, minus the wine and golf. Presumably.) Recent accounts about the five young men from Virginia who were apprehended in Pakistan as they sought to join Al Qaeda dramatizes this idea -- the men appear to have been rejected by the terrorist group on the grounds that they did not have sufficient credentials.
The club model of terrorism explains why terrorist masterminds are rarely betrayed by fellow members in the group, who stand to gain enormous financial rewards by betraying their masters. Unlike book clubs and other associations that make it easy for people to join and easy to leave, exclusive clubs are difficult to join and have high dues that make it difficult to stay. New members are rarely admitted unless they are able to demonstrate extraordinary commitment -- often over long periods of apprenticeship -- or have deep family connections with existing members. Exclusive clubs therefore produce very different kinds of human behavior than associations that are easy to join and easy to quit. They produce intense bonds of loyalty and tend to be extremely insular. It is these relationships that set the stage for the small group dynamics that can rewrite norms of human behavior, and allow people to carry out actions that seem insane, depraved or bizarre to those of us on the outside.
These ideas are discussed in greater detail in the chapter called "The Tunnel" in The Hidden Brain.