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There's a broad assumption is a lot of self-selecting organizations (Like DSA, Sunrise, Indivisible, etc) that participation in movements is largely a direct consequence of prior political beliefs, and we need to ruthlessly purge that conception from our brains.
I'll come back to this after i've had a chance to marinate, but basically What Organizations Do Matters in terms of who gets involved, and personal historical biography matters even more. Idealism is a bad theory for explaining movement participation
The must-reads on this subject are in no particular order "How Organizations Develop Activists" and "Moved to action" both by Hahrie Han, and "The Making of Pro-life Activists" by Ziad Munson. @jonathansmucker also traces some of this stuff in Hegemony How-To, but not
fully articulating that is one of the few weaknesses in that book. (It does a good job explaining the life cycle of the social formation but not the life cycle of a person's participation in that organization or movement)
There are lots of hard questions with ad hoc answers in organizing, but this is one thing where the evidence (both quantitative and qualitative) is strong enough where I'm confident that approaches that contradict the above are Just Provably Wrong.
