When a middle child - typically of a family of three kids who are close in age - feels left out or neglected.
Because the bigger sibling is the first born and typically an over-achiever, they are the most important and get the most privileges. A younger sibling is the 'baby' of a family and gets away with more as well as being the most looked-after. Middle-child syndrome starts when the middle child is squeezed between these two and have trouble finding their 'niche' in the family.
Middle children will sometimes...
-misbehave to get attention
-go with the flow
-work as little as possible
-be less parent-dependent then their siblings
-become the 'loner' of the family, not participate in family events unless told to.
-play the peacemaker
-be more likely to go to a friend for advice than a parent
-be very creative
In short its where the older child gets all the awards, the younger gets all the love, and the middle gets nothing.
(A real-life example of what happened to my younger sis, who suffers from middle child syndrome)
Oldest: Hey I just turned 16!
Mom: Lets go pick you out a car!
-1 1/2 yrs later-
Middle: Hey I just turned 16!
Mom: Oh.. er.. well we're still paying for your sib's car, so you'll have to just borrow from her when you can..
-2 yrs later-
Youngest: Hey! I just turned 16!
Mom: Yay! And I just finished paying for the other car! Lets go get you a car!
Middle: Hey! I should be the next one to get a new car!
Youngest: but its MY birthday
middle: Thats stupid
youngest: MOMMY SHE JUST CALLED ME STUPID!!
mom: Be nice to your little sister on her birthday!! Come on, birthday-girl! Lets go get that car!