Have you seen this? http://www.msnbc.msn.com/id/19373032/site/newsweek/
Many write ups of this study assume or imply that the firstborn is smarter than his siblings. One of the reasons I saw was that possibly because the elder tutors the younger and so gets smarter. (I searched my memory banks for references to my elder brother tutoring me and have come up quite empty) Shall I say "Ehhh-no"?
Unfortunately no one considered what the data might really be telling the researchers. The real effect is that having an elder sibling makes you dumber. This study isn't about my parents mistake because they happen to pop me out 2nd instead of first. It's about how my elder sibling's very presence is the cause of my stupidity and the reason my parents thought (think) "my elevator doesn't go all the way to the top".
Hmmm...that should put some new light on this study. :) :)