First and last children have an identity by the nature of their position.
An only child has an identity because it avoids sibling rivalry but has to face other difficult issues within family life on his/her own.
The middle children however many there are have to form their own identity within the group. This may be, the joker, the rebel, the sick one, the athlete, the academic, the good girl/boy, the quiet one or the attention seeker.
All these defenses are formed in order to gain special attention from the parents or parent.
When these children become adults many of these learnt behaviour patterns repeat throughout their lives, sometimes making relationships difficult to manage. An interesting experiment which can be reassuring and helpful in a group is to form sets of same place children who then share their experience of what being that number felt like.
The feeling of being alone becomes a shared one and enables management of difficult issues to feel easier.