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Not an issue with this study, but possibly worth noting that, naturally, there must be more firstborns than subsequent-borns (not counting the generic middle-child grouping). This could lead to a larger preceived effect.

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The survey asked for numbers of older brothers, younger brothers, older sisters, and younger sisters. The main analysis on the post is for people with one sibling, and there are exactly as many people with one sibling whose sibling is older as there are whose is younger.

Again, not talking about the study, just the general notion that first borns/only children are more successful.

Actually, it would be impossible for there to be more firstborns than subsequent-borns (barring death) because siblings require 2+ people, of which one and only one will be firstborn.

What if the number of only children is greater than the number of third children?

For every second born, there must be a first born, but not vice versa. For every third born, there must be a second born and a first born, but not vice versa...

So you're more likely to encounter a first born in the real world than any nth born, since some families will have only had one child.

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