Suppose the birth-order effect is caused by parents' dis=inclination to have additional children after having a dull child (and being more inclined after having a smart kid). Could that be statistically detected in these kinds of studies?
Norwegian sibling control study based on army data (900k) finds birth order effects on IQ, and these vary by generation aligned with the Flynn effect change. 
Source only in Norwegian apparently (thesis).
https://www.duo.uio.no/handle/10852/17989 …pic.twitter.com/otHjDtHx5X
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Good question. I suspect the answer is yes because one can predict presence of sibling X conditional on sibling 1 (...) traits, as well as parental traits for any interactions. E.g. parents that have mentally deficient kid may not feel they want another, too much work.
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Very often people do not wait that long until they have another child. Unless there is a condition, you can’t say that much until the kids are 12-13, even then...
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Yeah good point. So probably only obvious anomalies visible in the early years would have an effect.
End of conversation
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