The results of this study confirmed what I’d long suspected: First-borns are know-it-alls and the root of the problem is the way in which their parents treated them as children.
I'm an only child, so I've got no horse in this particular race. I've spent years listening to: eldest children complain about being put under too much pressure; middle children kvetch about feeling ignored; and youngest children whine about never being taken seriously. They do all this while telling "singletons" like me that we're selfish, self-centered, and spoiled. So, I'm unbiased in the sense that I think all three groups are full of it.
(For the record, having your parents' undivided attention is a double-edged sword. You do get a lot of encouragement. There's also not a whole lot you can get away with, and it's not like you can ever blame anything on anyone else. If something's broken, it's obvious that you did it. If something didn't get done, it's obvious you didn't do it.)
Anyway, the study itself seems to highlight some important things about family dynamics. In the case of eldest children, they may "benefit" from somewhat higher expectations placed upon them by their parents. They are often called upon to tutor their younger brothers and sisters.
Family dynamics are important. I once dated a woman who claimed (only slightly sarcastically) that her parents had managed to raise 3 only children.
Family "roles" are often self-fulfilling prophecies. People do, within certain limits, tend to live up or down to the expectations of others. If you're told often enough at an early enough age that you're the responsible one or the smart one, there's a good chance you're going to act the part to the best of your ability.
I was heartened by certain aspects of the study. As an only child, I can now blame my parents for deciding not to have any more kids. This has cost me 3 IQ points. More importantly, their selfishness denied me the opportunity to develop important skills in terms of mentoring and teaching. Alternatively, had I been a younger sibling, I might be more inclined to take risks. Finally, their selfish (or prudent, depending upon one’s point of view) decision not to provide me with siblings has also fostered the sort of spoiled, self-centeredness that would cause me to interpret the study in this manner. Who knew?