I think it has more to do with how easily manipulated a person's emotions are. I know allot of witnesses who I would consider very intelligent, and a few I consider geniuses. They are able to function well in their fields ranging from engineering to the medical field. HOWEVER, they still do mental gymnastics to prove the 'truth' to themselves and others, saying and doing illogical things that their belief system requires. Believing illogical things like the earth being only 6000+ years old, and that 8 men in New York are God's direct channel to direct his servants because THEY KEEP SAYING THAT THEY ARE.
It is more of a set of factors which can be classed as delusions / emotional illness:
1. Getting duped into believing that they COULD never die.
2. Being afraid to admit that they were duped.
3. Being convinced that anyone who does not believe what they believe must be 'mentally diseased', possessed, not appreciative of life.
4. Being duped that all wickedness will magically disappear.
5. Being duped into believing that their failing bodies will magically become 'perfect' without exercise or diet if they just listen to a set of old men in New York and help spread their propaganda to others.
6. Being duped into believing printed paper (Watchtowers, Awakes, other literature) has worth far beyond anything imaginable. The same way people are deluded into believing they should kill and prostitute for money. Which is just paper which some people have been conditioned to value beyond their personal value.
It's an emotional complex in my opinion. Feelings have the power to over-ride logic.