Serious Question. Has there ever been a study of mental illness and the using/buying into MLM's?

Hi all,

I am seriously wondering this: has anyone ever done a study of how mental illness may or may not play a part into involvement/usage of MLM's (and/or their products)? I notice in quite a few posts by various 'Huns' that many have either wording/verbiage that refers to mental illness, or there are other clues from pictures and such. Quite often the most vulnerable are the biggest victims. I have to admit to feeling sadness and empathy for those who get sucked into MLM's AND seem to have mental issues. What are everyone else's opinions/thoughts?