Of all the theories that claim to explain human behavior I have found Transaction Analysis the easiest to understand.
Explaining personalities in terms of relative size of 'parent', 'adult' and 'child' is easy to grasp. A child brings spontaneity, creativity and unadulterated emotions - the felt concept of life. A parent brings a sense of authority, nurturing, criticism - the taught concept of life. An adult is our ability to think and determine actions based on data - it is what brings in the thought concept of life.
Relationships and manifested in interactions between various components in different people. Intimacy, conflict, dependence, games, all are explained as interactions between adult-adult, child-child/parent-parent, child-parent or ulterior transactions.
I have observed for quite some time a pattern and correlation between a person and his/her birth order. Only now it occurs to me that probably the reason why the birth order has an impact on a person is because it determines how dominant are the parent/adult/child in our personalities.
When a couple has their first child, they are still unsure of what to expect. Their behavior is tentative, they strive hard to become the world's best parent. They want everything to be perfect. The child is always in the spotlight, her behavior under a scanner, to be analyzed to the minutest details. Under close supervision which inevitably leads to excessive nurturing and/or criticism, the child develops a big PARENT component.
The opposite happens in case of the youngest child. The parents by now are seasoned pros in child rearing and no longer fret over an upset stomach. They are no longer panic at the child starting late to walk and no longer work hard to make sure the child has mastered the alphabets in their second year itself. They are more able to enjoy the child as a child. Without any pressure on either themselves or the child to prove that they are capable. The child on account of elder sibling and (by now) more mature parents is pampered. As a result of being the baby of the family, child has a big CHILD component in their personalities.
The in-between children are of course more balanced. And not to forget that a lot depends on the parents themselves and the unique circumstances of the child.
Neither a big CHILD nor a big PARENT component is good/bad in themselves. They just bring a different quality to the table.