Only children & middle children are outsiders & the others as far as birth orders go. Both birth orders are unconventional so to speak. They are considered to be beyond the pale. They are indecipherable & misunderstood. Negative connotations are attached to both the only child & middle child as opposed to the so-called normative birth order. They are demonized to a greater extent than the normative birth orders as the oldest child & youngest child.
Only children & middle children are also highly individualistic birth orders. The only child because he/she doesn't have to compete w/siblings & is valued for his/her uniqueness while the middle child has to BE because he/she is lost in the familial shuffle, not valued for himself/herself but is always the appendage of the oldest &/or youngest sibling.
People also find only children & middle children to be strange & beyond the accepted birth order paradigms. Who/what are THEY really comes to many people's minds. One really cannot put a finger on these birth orders. To many people's minds, these birth orders would be analogous to the D group in junior high & high school-the odd kid out,outcasts to be pitied. In other words, the only child & middle child are UNPOPULAR birth orders.