For anyone who doesn't know the path to the Ph.D., it basically consists of a long slog in a narrow field. You gotta read everything (almost) that has come before you in the field, and then add something original of your own to the field. The point to the Ph.D. is to become a researcher in your field, and advance knowledge in the area.
I want to highlight a few personality traits or attributes that are necessary to get a Ph.D. in most cases. I say in most cases because there are a few exceptions, as always. The first trait is curiosity. This is an absolute requirement because otherwise you will get bored, tired, and walk away from the program. It takes a long time to go through all the history in a field, and it can get tremendously boring if you aren't curious enough.
The second trait is discipline. This tends to go along with curiosity, but it's the trait of being able to sit and concentrate for long periods of time and think deeply about an issue. The curiosity motivates you to look at the topic; the discipline keeps your butt in the seat.
These two traits, discipline and curiosity, are the only necessar traits to get a Ph.D. in most fields that I'm familiar with: humanities and social sciences. The hard sciences, especially those that are highly mathematical, require a the third: a high degree of quantitative intelligence. You need a lot of horse-power to crank through all those equations. Not to say that you can't learn the math if you're not terribly smart, but it is to say that mastery of the field requires a special level of mathematical intelligence and discipline and curiosity.
By and large, though, most Ph.D. holders are not especially gifted people - they have an above average level of discipline and curiostiy for their topic, however. This leads us to be especially well-versed in our own narrow field, which is desirable, since that's the knowledge we're trying to build. However, and here's the point of this post, getting the Ph.D. often leads people to believe their own hype, and think they in fact are quite intelligent.
This has gotten to be long enough, and I'm in the habit of writing lengthy pieces. As it is my first post, I think I'll leave off here, and pick up this topics again later.