"First Borns" have it all. They start out as onlies scooping up that first flurry of attention from their enthusiastic new parents, and when their siblings come along they maintain their winning trajectory thanks to parental biases and the diminished share of attention bestowed on their younger siblings by stressed out parents. Consequently they are more intelligent, more focussed on successful careers and more likely to succeed financially and earn more money. And, the greater the number of younger siblings there are, the bigger the disparity between the first and last.
Theories surrounding 'First Born 'advantage have been around for decades if not centuries.