There might be some that believe 100%, (or close to it), but not the majority. Most learn techniques to deal with their cognitive dissonance very early in life, and things like starting businesses and getting an education are just some of things they do along those lines. Clearly those actions do indicate that deep down they really do not believe, however the psychological and social impact of allowing themselves to see that fact, is usually at too high of a cost to motivate them to action. The obvious cost being not only loss of friends and family, but even more importantly loss of self identity. Anyone who has been through this process knows what I am speaking of.
The above is why it almost always takes some event in one's life to wake them up. An event such as a divorce, or some injustice by the elders tends to lower the "cost" of being brave enough to start questioning. The cost is lowered because social damage is almost always done to a person whom has experienced such an event. Sides are taken in the congregation, and the individual is often ostracized at some level. It simply becomes a matter of "I have nothing, (or very little), to lose by looking into my doubts".
I would almost go as far as saying that unless some faith dislodging event does take place in one's life, it is almost impossible to leave on one's own volition 100%. It may not necessarily be a personal matter, (such as a divorce or JC), but any one thing that causes enough discomfort, is what is required to leave at a bare minimum. I doubt anyone leaves that is psychologically and socially content, no matter how intelligent or how much they really don't believe deep down. It is not necessarily a matter of laziness, but rather, (existential), fear.
d4g