I don't think it's so much a matter of intelligence as it is of emotional dependency. I think most Witnesses who aren't born into the religion get sucked in in a moment of weakness, whether they are going through hard times or just a period of neediness. Everyone wants to be wanted, accepted and loved. Even very smart people will ignore unpleasant realities for the sake of these things. The problem is, once you ignore them long enough, especially in the context of JW's, the congregation becomes your life, your reality, your friends, your family. To leave then means not only to be ostracized by all of them, but it also means you have to face up to the people you left behind when you joined, if they are even still there. If they aren't there, it's like you just moved to a new city by yourself, and have to start over. If they are there, you are always going to be "that guy that used to be in a cult". Even if this really isn't the case, which it may well be, it will play out in your head that way.
If you start believing the drivel in the Awake! and Watchtower magazines and their various other literature, it's even worse. I think they've done experiments that have shown highly intelligent people to be more susceptible to brainwashing techniques than less intelligent people, although I can't be sure where I've seen this. Not in the Awake!, I would venture to say. The methods they use are obviously very effective--it's easy to overlook the redundant use of phrases like "Clearly this shows . . . ", "Obviously . . . ", "Apparently, then . . . " followed by one of their peculiar ideas, but they stick in your subconscious. The way they redefine words like "truth" and "loyalty" such that the connotations they have in everyday life project onto the WTS versions of the same. The way the "study" questions in the footnotes in The Watchtower are designed to make you repeat word for word the corresponding sentences in the paragraphs you are being questioned about, fooling you into believing you are thinking for yourself--it's all designed to chip away at your defense mechanisms. If you go to all the meetings and hear the exact same thing pounded into your head over and over again, day after day, week after week, year after year, you WILL start to believe it. It might even be worse for intelligent people, because they have always considered themselves to be able to think critically and logically, and it is an extremely hard and humbling thing for them to admit they are not still doing so, in fact that they have become walking, talking JW literature.
As far as born-in's go, it is doubtless a different situation. Being raised from childhood as a Jehovah's Witness, as I was, you know nothing else, and it is very difficult to let go of the only thing you've ever known. I was fortunate to have not very devout parents, who stopped going to meetings shortly after I was in grade school, although I continued going to the Hall and out in service on the weekends with my grandparents until I was about 14. Even so, I could not see JW's for what they really were for a long time. I kept making excuses for them, saying they were not that bad, defending them. That's the effect they have on you. I never see my extended completely JW family anymore--I'm not vey close to them anyway, but I think I finally know why. Of course, being brainwashed and being told not to think critically when you have not known anything else makes it doubly hard to leave, regardless of your intelligence. It takes a very strong person (and sometimes years of therapy) to go from being a fully indoctrinated Jehovah's Witness to being free from their influence.
These are all things that play on universal human needs, emotional needs that have little to do with intelligence.
