Middle child syndrome?

My sister and her husband have two daughters.  My nieces are 7 (almost 8) and 5.  They are expecting a new addition to their family - a third baby girl - to arrive in June of this year.  My seven year old niece is excited and can't wait to meet her new sister.  My five year old niece is slightly less enthusiastic...In a few short months, she'll no longer be the baby in the family; she'll be the middle child. 

My parents also have three daughters and I happen to empathize a bit with my five year old niece; I was just shy of five years old when my younger sister was born.  In one of our family albums there's a picture of the three of us (my older sister, my younger sister, and me) on the day my little sister was born.  My older sister is holding her and smiling.  I'm sitting next to them, scowling.  I remember that day.  My parents told me I wasn't old enough to hold the baby.  I was pissed.  It's a longstanding joke in my family that this incident set the tone for the rest of my childhood. 

Maybe you've heard of something called the middle child syndrome.  Well, if such a thing truly exists then I had it, no question.  I can remember feeling like I had no place in our family.  My mother confirms that I told her as much, repeatedly. 

As an adult, it seems odd that I resented being the middle child.  I love my sisters and I'm glad there are three of us.  We each have our role in the family.  We have a close relationship with one another and with our parents.

If you ask me how being a middle child shaped my personality I'm not sure I could explain it.  I know I went out of my way to be as different from my older sister as possible because I wanted to be my own person and not just "Tina's little sister."  I remember wanting to protect my younger sister; I was quick to go after anyone who was bullying her.  Thinking about it now, it seems like I got the best of both worlds by being both an older and a younger sibling.  Yet I know I didn't always feel this way.

There is a large body of research exploring how birth order affects everything from intelligence to education attainment, from personality to perfectionism.  While traditional wisdom accepts that birth order has a strong impact on who a person becomes and what he achieves, the research is inconclusive.  Some studies suggest that birth order has a significant impact on a range of other variables; other studies find no evidence for this at all.

What I really wanted to know when I started looking into this topic was this: Is "middle child syndrome" a real thing?  The short answer is no.  There is no empirical evidence to suggest that as a whole middle children tend to feel lost, neglected, confused, or resentful (or at least not with any more frequency than other children).  So empirically, scientifically, professionally speaking, middle child syndrome does not exist.  The evidence simply doesn't support it.

Yet a quick Google search of the term "middle child syndrome" indicates that while there may be no empirical evidence to support it, society readily accepts it as a valid phenomenon.  It is a cultural myth.

And yet, don't these things tend to contain some kernel of truth?  What do you think?