Social and economic deprivation is not the root cause, otherwise explain why educated students, doctors and engineers are joining IS?
How many are? Do we have any good information on this? I doubt IS releases the professinal and educational backgrounds of its hires!
Generally, my impression is that historiclly the middle class has provided the idealogues; the theory is thhat if you're living hand-to-mouth you have more immediate worries and less time to study such things, and if you are illiterate you have other obstacles. That's just one possible but commonly repeated hypothosis: I don't know what basis there is for it.