But by that logic younger siblings would be overrepresented next bin, [30,40]. What goes up must come down. Personally, I mulled over it for a while and I don'

But by that logic younger siblings would be overrepresented next bin, [30,40]. What goes up must come down. Personally, I mulled over it for a while and I don't see any mere "statistical" effect that's plausible. I think we are forced to conclude there's a psychological difference with firstborns.