What does "member retention" mean here?

Context:

Deprogramming and the anti-cult movement[http://en.wikipedia.org/w/index.php?title=Mind_control&action=edit§ion=6]

Over the years various theories of http://en.wikipedia.org/wiki/Religious_conversion and member retention have been proposed that link mind control to http://en.wikipedia.org/wiki/New_religious_movement, and particularly those religious movements referred to as "http://en.wikipedia.org/wiki/Cult" by their critics. These theories resemble the original political brainwashing theories with some minor changes. http://en.wikipedia.org/wiki/Philip_Zimbardo discusses mind control as "the process by which individual or collective freedom of choice and action is compromised by agents or agencies that modify or distort perception, motivation, affect, cognition and/or behavioral outcomes",^[32] and he suggests that any human being is susceptible to such manipulation.^[33] In a 1999 book, Robert Lifton also applied his original ideas about thought reform to http://en.wikipedia.org/wiki/Aum_Shinrikyo, concluding that in this context thought reform was possible without violence or physical coercion. http://en.wikipedia.org/wiki/Margaret_Singer, who also spent time studying the political brainwashing of Korean prisoners of war, agreed with this conclusion: in her book http://en.wikipedia.org/wiki/Cults_in_Our_Midst she describes six conditions which would create an atmosphere in which thought reform is possible.^[34]