Well, as is obvious from all the other posts, it depends on the personality of the child and all other family members.
I am the middle. I have two brothers, fours years apart on either side, so I am the only girl. I am very happy, happily married, "well-adjusted" you could say, I suppose.

We are planning on stopping at one child, but not because of any drama in my own childhood. I think my parents did a great job of supporting each of us siblings, celebrating our different strengths and spending time with each of us individually. We are all vastly different people.
I think it is very possible to have a happy, well-adjusted middle child if you are a happy, well-adjusted family in general. And, of course, sometimes the very best environment and support can still not be enough if the person is "destined" to be unhappy for some other reason. And similarly, you can be happy and well-adjusted even though your family life was one big soap opera. It's not necessarily nurture over nature.
