I agree with you, Laura. I've witnessed it, too. Time and time again. Birth order has a clear impact on how we are parented and thus, how personalities develop.
In my family, every child is a classic archetype of his birth order status. Same for my husband's family and our children. Friends and colleagues share the same birth order stories about their families.
I've noticed, however, that some extenuating circumstances can factor in. For example, sexual abuse, neglect, mental illness can change how children view themselves and the birth order roles. If there is a handicapped or disabled child in the family, that also affects birth order roles.