I wanted to say the same: parents don't treat all children the same. For example, I have the feeling that the first child is the "practice" child. The parents learn from the mistakes made with them and don't repeat them with the children that follow. I don't know if there's any research to back this up and yes, I am a first born.
I think sequel kids, more than anything, benefit from having a trailblazer to refer to. It's no doubt true that parents get better at the job, but kids learn from demonstration. Older sibling is hypersensitive and has a hard time keeping friends around -> I better learn to swallow my pride. That kind of thing.
I've observed the same - unfortunately, first time parents are forced to try out all kinds of parenting experiments on their first-born, before they figure out how to be "good" parents. And subsequent kids, especially if they have them after some gap, get the benefit of this experience. Add to the woes of the first-born, they not only have to deal with normal sibling jealousy (of having to share their parents affection), but also resolve the emotional issue of why their younger siblings have an "easier" time (i.e. why their parents treat them "differently").
