I had noticed something similar in high school.
Virtually all of my friends were either first born, only children, or born after a significant period of time after the previous child. Almost without exception, we excelled over our younger siblings academically, in success in careers, and in a marked loss of “screwing up” in life.
This carried over into college and into general life choices. While not every girl I dated was in the same category, looking back, my most compatible relationships were with women with the same quality, including my wife.
Maybe it’s all coincidence. Maybe it has something to do with a few years of undivided attention from parents. Maybe it’s driven by parent expectations. But it seems to indicate some cause and effect, at least in my experience.
“Maybe it has something to do with a few years of undivided attention from parents”
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MY first born got only 12 months of undivided attention,I very much doubt that it had a great impact on her intelligence.
Of course there are tons of exceptions, but I have seen this in horses, where the first one was a big time performer, the later ones were often good, but not superstars.