Haha, I love this question, as a middle kid. To some degree, I think it has validity, because when parents have their first child, they seem to be in hyper-parent mode (overly protective, since they don't know what to worry about and what not to worry about, coddling, devoted, etc). When the second kid comes along, they tend to be more laid back. But when that third kid comes along, it does sometimes seem that the middle kids ends up by the wayside. They never experience the hyper-parent, and then their normal parenting is a little cut off by the baby.

I'm a middle kid, my mom was a middle kid, my dad was a middle kid, and my boyfriend is a middle kid. I would saw that many middle children I have known do fit the stereotype a little, but there's a flip side to being a bit alone: independence. Most of the middle kids I know are also really responsible adults. We learned to cook for ourselves, take care of the younger sibling, get our homework done without a hovering parent, etc., because we had to. I personally think it's a fair price to pay for being self-reliant.

My own personal theory is that what is more important than birth order is a birth order/gender combination. I think it's one thing to be a first born, but quite another to be the eldest daughter (even if that makes you a second, third, or fourth born). Because our society still has echos of "women should take care of men" and because mothers often lean on their daughters for help around the house, I think eldest daughters have a special role, apart from their birth order. They can sort of expect to be the ones who care for aging parents, be responsible for their siblings, etc. I'm sure eldest sons feel some pressure too, especially if their culture demands heavy responsibility, but frankly, I wouldn't know.

Anyway, just my opinion, but I definitely think gender in the family plays a huge role.