"...there seems to be little correlation with siblings, at least beyond what you would expect from having parents who emphasize learning math."
I'd bet for a set of negative factors. For you to be 100%, you need to have full 15% of genes (maybe there are a lot involved, so it could be very improbable to have more than 10% of that 15%), 30% of early nurture and 40% of favorable social conditions (good environment, or just not being poor that lets out a big part of humanity) and the rest of luck, effort and good teachers.
The lack of any of these conditions could explain why the siblings are not so bright. The lack of genetic conditions and the early nurture could also ruin more easily the posibilities.
"Genetic" is also broad. It could be quicker thinking, focus, even a illness that forces a child to stay home reading instead of playing outdoors.
I'd bet for a set of negative factors. For you to be 100%, you need to have full 15% of genes (maybe there are a lot involved, so it could be very improbable to have more than 10% of that 15%), 30% of early nurture and 40% of favorable social conditions (good environment, or just not being poor that lets out a big part of humanity) and the rest of luck, effort and good teachers.
The lack of any of these conditions could explain why the siblings are not so bright. The lack of genetic conditions and the early nurture could also ruin more easily the posibilities.
"Genetic" is also broad. It could be quicker thinking, focus, even a illness that forces a child to stay home reading instead of playing outdoors.