http://www.education.com/reference/article/influences-sibling-relationships/ said:
Success seems to fit firstborn children. Many firstborns show leadership qualities. Alfred Adler said firstborns were in a favorable position being larger and stronger, but, to keep their position, they also had to be more clever (1928). In studies as early as Galton's English Men of Science (1874), disproportionate numbers of firstborns have achieved eminence. A higher percentage of firstborn children have become scientists, professors, presidents, Rhodes scholars, and astronauts. More firstborns have been finalists in the National Merit Scholarship tests compared to any other birth order (Muzi, 2000). This advantage may be explained by the fact that firstborns have only adults for language models and social interactions in the most formative period while their siblings are influenced by their predecessors in the family.