On the positive side, middleborn children are good negotiators and adaptable; they often feel little need to "control."1996, Gary Smalley, Making Love Last Forever, Thomas Nelson, →ISBN, page 182:
middleborn
noun
Etymology From middle + born.
1
The middle child to be born to a parent or family.
A middleborn who feels left out may become unsure about his place in the family.1986, G. Hugh Allred, Teenager: A Survival Guide for Mom and Dad, Bookcraft, →ISBN, page 36: