Dr. Kevin Leman in his The New Birth Order book shares a typical pattern of how children turn out based upon birth order; it is by no means without exception. Here is a typically scenario in a family of three children not too many years apart…
Usually the first born is a bit of a perfectionist and very conscientious.
The second born is a mediator, has many friends, and is a bit of a maverick.
The third born is a people person, charming, and seeks lots of attention.
An only child is a little adult by age seven and self-motivated.
Between inherited personalities, birth order, and parental expertise (that improves with experience) we naturally cannot treat our children exactly the same-- but fairly.