Me and friend have understood this instinctively long time ago. However we could not really discuss it with anyone else because we were seen as too PC and too horrid. But we thought, there is a reason several cultures have valued the firstborn, not just for social factors, but for epigenetic ones.
And look, we were right. Again.
We are both the second kid.
She is gay but that most likely doesn't have anything to do with birth order since the gay thing is only in boys.
But lets look at the other things.
We're both shorter than our siblings. She is not a lot shorter, I am.
Her sibling has performed academically better at school. My brother is also officially tested, and has 10 points on me.
I remember being a kid, a very young one. I learned from my brother because it was easy. He had to teach all those things to himself. Maybe why he became wealthy and influential while I can barely put food on the table.
But... there is more. Both me and my friend have ESP. Our siblings lack that. And if you don't believe in that. We both have creativity. Our older siblings have... well none. They are not good writers and they can't paint or draw worth shit. We can. We had art exhibitions an sold our work. They rarely have novel ideas. They can only be smart in the fields where everything is already set.
Also, we are mentally stronger. As we were left to ourselves more emotionally, we have become able to reward ourselves, while our siblings cannot live without external award.
I wonder if there is a health factor too. Our siblings have stronger health than we do.
I expect biology to be the biggest factor. The social factors of being the second child I think do exist to a small extent, and by tradition it is seen as social. But I think that thinking is wrong. I think the uterus as an environment IS the bigger factor.
Thanks for the "article". It sparked things I have thought about prior and I will have new things to tell my friend.