In the last week a number of news outlets, including the venerable New York Times, featured a study by Norwegian researchers claiming that firstborn children tend to score higher on traditional assessments of intelligence (I.Q. tests) than non-firstborns. The researchers are quick to point out, however, that it's most likely a difference in the way parents treat their children than anything biological. For example, parents often rely on older children to mentor younger siblings; this mentoring may in fact be more beneficial to the older child than to the younger ones.
As the New York Times points out today, people like studies like this because they seem to validate stereotypes we already believe about sibling dynamics, i.e., stereotypes about "responsible" firstborns, "rebellious" middle children, and the like.
Research like this only goes so far, though. Take me, for example. I am a youngest child and my siblings will tell you I have many of those qualities. Meaning, of course, that I am pampered and spoiled. But because of a pretty big age gap between me and my siblings, I also have many qualities of a firstborn or even an only child. I don't know what this means "scientifically," but I do know that it was a pretty interesting way to grow up.