"Birth order refers to the order a child is born in their family; first-born and second-born are examples. Birth order is often believed to have a profound and lasting effect on psychological development. This assertion has been repeatedly challenged.[1] Recent research has consistently found that earlier born children score slightly higher on average on measures of intelligence, but has found zero, or almost zero, robust effect of birth order on personality.[2] Nevertheless, the notion that birth-order significantly influences personality continues to have a strong presence in pop psychology and popular culture.[3][4] " from:

https://en.wikipedia.org/wiki/Birth_order

This is a wonderful theory, that really makes sense to me. Wish I had an older Sister to look up to growing up.