JAM:
I think it is a combination of factors that will indicate why a person or family will leave the religion. And, it has nothing to do with intelligence but more to do with other circumstances. Two people can realize the religion is crap but maybe only one will leave. The other may have family and business connections and may feel trapped.
I only had myself to worry about and started my "fade" cold-turkey at an opportune moment. My only regret was that I didn't follow my gut and leave years earlier!!