I ran across this guy's stuff a year or so ago and found it fascinating. He asserts there are 5 birth order personalities:
Here is my EXTREMELY brief summary of the stuff in the link below:
The Only learns to play alone without feeling lonely, dislikes intrusion, and can be overly expressive via the positive feedback created by the tantrum.
Each first born starts as Only and if there is a perceived loss of love by all the transition when the Second arrives, the Only transitions to First and becomes the "good" kid or the performer to win back love.
The out-performing of the more developed First creates a coping mechanism in the Second to choose perfectionism in certain areas, critiquing others, and suppressing feelings.
The Third copes with the critical second born by being strong and becoming fearless. The Third who fails becomes fearful.
The Fourth responds to the rejection of the Third by trying hard.
The Fifth is ignored by the Fourth and has Only personality...and it starts all over.
Birth Order Plus - Intro to Birth Order
I've noticed that Third Born has a high correlation to ENFPs. I think Onlies are more often N. Recently reading an ENFJ thread, there seems to be some correspondence to First traits.
Partial Thread Tally: (not being updated any more - I couldn't keep up)
INTJ,INTP,ENFP,INFJ,ENTP,ISTP,ENFP...(seem to be a majority)