  Extremist ideologies are finding new homes in online forums. These serve as
both places for true believers, and recruiting-grounds for curious newcomers.
To understand how newcomers learn ideology online, we study the Reddit archives
of a novel sexist ideology known as the "the Red Pill''. Matching a
longstanding hypothesis in the social sciences, our methods resolve the
ideology into two components: a "behavioral'' dimension, concerned with
correcting behavior towards the self and others, and an "explanatory''
dimension, of unifying explanations for the worldview. We then build a model of
how newcomers to the group navigate the underlying conceptual structure. This
reveals a large population of "tourists'', who leave quickly, and a smaller
group of "residents'' who join the group and remain for orders of magnitude
longer. Newcomers are attracted by the behavioral component, in the form of
self-help topics such as diet, exercise, and addiction. Explanations, however,
keep them there, turning tourists into residents. They have powerful effects:
explanation adoption can more than double the duration of median engagement,
and can explain the emergence of a long-tail of high-power engagers. The most
sticky explanations, that predict the longest engagement, are about status
hierarchies.
