In some ways, it makes a lot of sense that a firstborn will approach life far differently than a middle child, singleton, twin, or last born. The people who started researching the idea believe it is very important and that certain traits are universally to be expected. For instance, a firstborn should be more conscientious, more rigidly adherent to rules, and mature earlier taking on more responsibility. A baby should be more rebellious, creative, and less observant of rules. Okay, makes some sense when you consider it, right?
There are problems with the theory, though. First, research has failed to fully support the conclusions. Second, far too many fail to fit the mold. I'll use myself as an example since I know me well. I'm a last born. By the books, I should be rebellious and creative. Check. Got that right. Problem is I'm also super conscientious, hyper-responsible, and tend to take on leadership roles more appropriate for a firstborn. Hmmmmm... Oh wait, say the proponents, didn't we mention that age differences between siblings, gender, stressful episodes during childhood, early loss of a parent, and relationship to your parents also has an effect?
With all those influencing factors, you really want to tell me that birth order determines your personality? I think not. Why don't we just admit that personality, like so many things in life, is shaped multifactorially.
Or maybe I'm just a really messed up last born...