Christopher

“Lacking older siblings, the oldest or only child identifies primarily with her parents, conforming to their ideals and demands, not the least reason being that she no one with whom to share those demands. Since firstborns try to live up to the expectations of adults- teachers' as well as parents'- rather than that of peers, they are likely to learn more and to bring home better report cards than younger siblings. Thus firstborns pave the way for younger siblings, setting the standards against which they are measured and measure themselves.

Middle children tend to be more gregarious and more dependent on the approval of peers than that of adults. For one thing they have the example of the older sibling- who has the credibility of generational sameness- to guide them in their decisions and to teach them the rules of the family road. An older sister who was grounded for a month for coming home late from a date, for instance, is a lesson not lost on her younger sister or brother.

At the same time younger children are buffered by birth order from their parents' sole concentration. Hence they are treated with more indulgence and are called upon less to take on responsibilities.”
― Victoria Secunda, Women and Their Fathers: The Sexual and Romantic Impact of the First Man in Your Life