Hi Jack. Nice to meet you.
The primary doctrine, the one that holds them all in captivity, is that the Watchtower was selected by Jesus in 1919 to be God's sole channel of communication to mankind in our day.
As long as they believe that, the WT can get them to believe just about anything.
Combining that belief with all seeing invisible space creatures spying on your behaviour day and night and an easily offended bi-polar killer god bringing Doomsday any day soon and you have a recipe for a nut job. Which I was for many years.
It doesn't matter if they are not a practicing, preaching JW going to the Hall every Sunday or a tatooed, unbaptised born-in who peddles drugs, if they don't fully understand that that doctrine is a load of bollocks, they are damaged goods and are likely to run back to the cult when the right triggers are pushed.
Cheers
Chris