I would say life experiences and environment have more to do with it than anything. Here is why. Unless there is something magical about being first born in a family....the expections put on first born makes them excel at being leaders.
THE FIRST BORN PHENOMENON
●Two-thirds of all entrepreneurs are firstborns
●Twenty-one of the first twenty-three astronauts were firstborns
●Firstborns are twice as likely as laterborns to become CEOs
●Fifty-five percent of all supreme court justices have been firstborns
●Over half of U.S. Presidents have been firstborns
HOW DO WE EXPLAIN THIS PHENOMENON?
●People have higher expectations for first borns
●First borns are given more responsibility
●First borns get more encouragement, feedback and attentions
http://www.legalauthorityblog.com/2009/05/the-first-born-phenomenon/