The Oldest: The first born child is kind of like the guinea pig. Parents are brand new to parenting and tend to be more by the book. They are more strict and protective with their precious first child. In turn, older kids can be overachievers, cautious, rule-followers, ambitious, bossy and controlling.
The Middle/Second Child: When the second child comes around parents tend to have a handle on parenting. They are more relaxed and less strict and this can influence a middle child's personality. Now if this child has a younger sibling they sometimes feel as if they are lost in the shuffle and tend to feel left out. This can lead to a middle child doing what they can to stand out even being a bit more rebellious and a rule breaker. For middle children their friends tend to be very important to them. They are more likely people pleasers and do not like to be told what to do. They tend to do what they want.
The Baby/Last Born: By this time parents are much more laid back when it comes to parenting. Or in some cases parents have just really figured out what works and what doesn't. Or they have given up trying. There are usually less rules for the baby of the family and less discipline. This laid back attitude is reflected in the youngest child's personality. The babies of any family are usually the fun-loving, out-going, spontaneus, and adventurous types.
As you can see it is more about the parenting and how parents relate to each child. When I think about my sisters and I we totally fit our birth order types. I am very much the responsible one, my middle sister lives by the "I do what I want" motto, and the youngest is the life of any party.
Do you fit your birth order personality?