If we include both daughters and sons, the privilege of the first-born becomes even more apparent: Reports suggests that first-born kids have higher IQs and get further in business than their younger siblings.
Not only that, their parents consider them to be more accomplished.
What could cause such a gap?
Developmental psychology has a smattering of explanations, though none of them are conclusive.
One dating back to the '70s is called confluence theory. The idea is that kids' smarts grow in accordance to the "intellectual culture" of their home - if you treat your kid like she's a small human, her maturity will grow accordingly. But as more children enter into the family vortex, that intellectual level goes down. In effect, the younger kids enter a more "diluted" intellectual culture.
Then there's the equity heuristic. According to this framework, parents want to invest the same amount of attention into each of their kids. There's a problem though: When you only have one child, he gets all of the attention. With two, it's halved. With three, even less. So while the intent is to invest equally, the result is anything but.
Also consider the no one to teach theory. All sorts of research shows that teaching something is the key to actually understanding it; older siblings, being older, have the opportunity to teach their young sibs, but the babies of the family have nobody to receive their wisdom.
Unhappily, there's the divorce theory, which observes that family traumas like divorce are most likely to happen after the first child has grown up a bit, making the family upheaval harder on the younger kids.
Even less comforting is the evolutionary argument. It states that each kid is vying for attention of the parents, but they do so in different ways - carving out niches like Darwin's finches. One kid's an actor, another a star student, another an athlete; call it "Royal Tenenbaums" syndrome. The privilege of the first kid, then, is that he gets to stake out his niche before any competitors emerge.
Which, as a younger sibling, I can report is totally unfair.