I think I saw recently that the first-born kid gets, on average, something like 3,000 more hours of direct time with the parents. As a result, the authors of the study believe that leads to higher intelligence scores and long-term greater career success, emotional stability, social acceptance, yadda yadda yadda.
The 2nd, 3rd, etc kids all end up getting roughly the same number of hours of direct attention from the parents (that is, 3,000 less than the 1st).
Personally, I don’t put much stock in such things… way too many variables, and way too many exceptions in real life.
Note: all that’s from recollection only, for example, the time difference might just be 300 hours. Whatever the details, the authors of the study believed the difference to yield significant difference in the children’s upbringing.
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Oh look, I found the article. Again, any mistakes above are my own, refer to the article for the real details.