That's possible, Risen. I am firstborn, and do find my sister to be a bit selfish and dysfunctional at times. My sister, second in the family, fits the pattern others have described on this thread, but my two brothers - third and fourth - are not selfish at all. They are even younger than her, but have not followed the same pattern. So in my case, it is strictly limited to the second.
Some other possibilities come to mind... the second receives lots of attention even while there is already another sibling present (due to being younger and needing more care), therefore this might create a perception of "I must be more special than the other" which turns into a sense of selfishness later on. Or maybe the second child is a chance for the parents to "do things right" after all the errors and regrets from raising the first, which swings too far the other way and creates a spoiled child instead.
But that's assuming environmental factors are the only ones. Metaphysical factors may also play a role, which is what some of the discussion above is implying. Even among infants and toddlers, some have more of a spark behind their eyes while others have far less. Even within the same family sometimes. I knew several examples of both during the early 90s while living in Iowa... recently I took a trip back and got to see what became of them. What I saw didn't surprise me. The ones with sparks became individualistic and sensitive pre-teens, while the others became neither. I think that whether someone has that 'something' determines whether they become molded by their environment or catalyzed by it.
Acquiring fringe knowledge is like digging for diamonds in a mine field.