Hmm, so this seems to be turning up a bunch of random results.
It's likely a contributor, but I think there's much more that goes into J/P preference than birth order.
"We grow up thinking that beliefs are something to be proud of, but they're really nothing but opinions one refuses to reconsider. Beliefs are easy. The stronger your beliefs are, the less open you are to growth and wisdom, because "strength of belief" is only the intensity with which you resist questioning yourself. As soon as you are proud of a belief, as soon as you think it adds something to who you are, then you've made it a part of your ego."
I'm an oldest J & my boyfriend's an oldest J. However, both of us grew up carrying a disproportionate amount of responsibility, so that may have contributed towards the J.
I'm the oldest of a single mother family, so I had to take the responsibility for my brother while my mother worked a couple jobs to pay the bills. My younger brother is a P. I am sure that factor contributed to the strong J as well as the strong auxillary Fe.
I forgot to mention the gender effect. If you have a male child followed by a female child, the female child will also take on some oldest child characteristics because she's the oldest female. She kinda gets to be in a league of her own for a lot of things.