As far as I’m aware the gold standard for estimating the heritability of intelligence are twins that have been adopted into different families. You compare the variance of monozygotic twins adopted into separate families to the variance of dizygotic. If there is greater variance between dizygotic twins you can conclude, since the monozygotic twins share close to 100% of their genes, that the diminished variability can be explained with their shared genes.
However, like I said, there are problems with this assumption, mostly from sampling bias. But you can also argue that the twins share environment while in the womb, the bacterial environment actually interacts with your genome so the shared genome is not actually 50% vs. 100% but somewhat lower. But my problem is actually related to the definition of intelligence. There is no consensus that IQ is an accurate estimate of intelligence, far from it.
However, like I said, there are problems with this assumption, mostly from sampling bias. But you can also argue that the twins share environment while in the womb, the bacterial environment actually interacts with your genome so the shared genome is not actually 50% vs. 100% but somewhat lower. But my problem is actually related to the definition of intelligence. There is no consensus that IQ is an accurate estimate of intelligence, far from it.