> Heritability is always measured in the context of Environment X. If you change to Environment Y or Z then the heritability will often change greatly.

That's not a very meaningful statement. If you took two twins and severely malnourished one of them it would not be useful to say: "See! IQ is mostly environmental!".

You have to assume some kind of baseline environment that nearly everyone will share, and that can be full-filled just by the virtue of growing up in a country like America. Otherwise, you are just concerning yourself with insignificant outliers.

Here is a twin study that places the heritability at ~80%: https://www.cambridge.org/core/journals/twin-research-and-hu...