Is your first child really the smartest?

I am the second child in my family, and I was a pretty smart kid: advanced classes, honors student, deans list - all that good stuff (you're welcome, Mom). So, I was really surprised to read that, overall, first-born children are actually smarter. Even more surprising, is that this is true internationally.

But why is that?

Don't get me wrong, my older brother is pretty smart. (I actually don't know if he and I conform to the study or not.) But I am curious. Is it because Mom and Dad are able to focus more attention on their first child? Is it because they are stricter and expect more from their first-born? Is it because older children have to grow up a bit faster to help teach and take care of their younger siblings?

I'm not sure, but this article by the Atlantic does a nice job of taking a look at the varying theories as to why first-borns are (generally speaking) smarter.

So, what is the situation for your family? Do you see a difference between your first and subsequent children and their school performance?