It can also depend on the sexes.
Eg, my Dh is one of 4, so technically there are two not one, middle child, or none if you look at it the other way!
Anyway, the order is as follows, girl, boy (DH), boy, girl.
I've spoken to my MIL and DH about this. If there is a typical "middle child" in their family it's the third child.
My theory is that the first born is the first born, so there's a ready made identity from the off, my DH was second born, but also the first son, so again, first boy blah blah, DH's brother was the third born, there had already been a one of each before him..no real sense of identity etc.
Incidentally, he's much more needy, less independent and drama-prone than the other three, so maybe there's something in it!
The fourth child was a DD, but is that even relevant (because perhaps that child was always going to be identified as being the youngest, which is sort of an identity in itself, if you're that way out, I suppose)?
It would have been interesting if my DH's oldest sibling had been a boy. Would that have impacted on his personality? Who knows? I do wonder, though!