Having carefully read the post I realise that a man's observations are just as relevant.
We have two children separated by 5 years. The eldest, our DD is worrier, even to the stage of tending towards being bi-polar, she has a very stressful job that pushes her too far quite often, but without that stress she is bored. We have always thought she just took after me, as that is exactly how I am. Our son is the opposite, whilst he also has a stressful job as a company director, he is so laid back and rarely shows sign of any stress, exactly like Mrs. P. We had always thought theses were traits inherited from us, but it could well be the birth order. Our DD children are also very different in a way that the birth order may have affected, ie her second child is the laid back content one.