There was a story on the Today show this morning (I couldn't watch it because I had to leave for work) about scientific support of the theory on how birth order affects personality, etc. I was somewhat familiar with this concept, & I would expect you've heard of it, too. I looked it up on Google, which is how you find out things, & there are a lot of places that talk specifically about this theory. I picked the reference to Wikipedia, & it was interesting to read what some have said. I'm the first-born in my family, & I have to say that what they said about the first-born, both pros & cons, sounded like a description of myself. How far can you take this, though? I know several first-borns who are very different from me, but I'm guessing they would also identify with this description.
The problem with this study is that the people finding this stuff out are themselves either first-borns, youngest, one of the middle-children, or only-children, & that would give them a bias on their opinions of their own & other people's categories. The only way to get an objective viewpoint on this would be from a scientist from outer space. Even androids or computers with Artificial Intelligence would be influenced by the bias of their programmers. This can apply to many arenas of study, can't it?