When Ralph McGehee was being interviewed on Australian Radio regarding the CIA's role in the elimination of Australian Prime Minister Whitlam, he explained part of it, based on how people are selected for the agency. He explains what's called the ERA personality scale:

They were looking for the total personality picture. They can pick up one element of your personality or another or another and come up with the individual's total picture. And what they are looking for is the E.R.A. type of personality. In the E mode they are looking for the extrovert who likes to be active, who doesn't particularly like to sit and think, who doesn't like to plan in advance but works by trial and error. In the R mode they are looking for the Regulated Individual, or the rigid I call it, a person who sees the world in black and white certainties, there are no shades of gray, a person who has trouble feeling sensitivity towards others. Then in the Social Mode they are looking for the A or adaptable individual, a person who will not protest, who adjusts his personality to the milieu he finds himself in at the time.
So you get this total E.R.A. type of personality which they are looking for, and you recruit this man, knowing this person, knowing his ideological position, always to the right-wing of course. And then you take this sort of personality, put him into the system, indoctrinate him in the tenets that the world is threatened by an international communist conspiracy and you feed him all the so-called intelligence the Agency collects and you have a sort of hermetically sealed lifestyle that reality can never penetrate. ...

I well suited the E.R.A. personality type, I guess, and I tried to hold off reality for several years but I was living in the belly of the beast and my defenses sort of crumbled slowly and it was a very -- at first you know, 'I can't believe this is true', 'I don't want to believe', and ultimately I had no other recourse but to accept the reality that I was involved in and this was, as you can imagine, extremely painful. Not only was the Agency that I had so given my life to but my country was deceiving and killing and all the things that I sort of pondered my personality and like all in the world shown to be deceit so it was extremely painful and, of course, one thought of the various solutions to one's dilemmas such as suicide which I ultimately rejected in the hope that I could bring my story out.