Actually there's a very classic theory about it, by Alfred Adler. Adler claimed that the firstborn child is given care and love without having to share it with his siblings. That situation changes when a second child joins the family, and the elder child realize that now he has to share the attention of the parents, with his sibling. Adler referred to 3- child family, and predicted that the middle one has most chances to develop and become a healthy grown up, as he never suffered from "loosing the crown", because he was the second, While the elder child has most chances to have problems in life.

For further reading- Adler's birth order theory .