But how can you distinquish between heredity and environment? You would have to have control groups of kids with smart heritages being raised by dumb folks. And I don't thin smart parents are going to go for that.
Identical twin studies. Studies with identical twins (with identical heredity) show that even when raised in different environments that they still score the same on IQ tests.
I believe they studied identical twins adopted into different families.
They compare identical twins raised apart with fraternal twins raised apart. IIRC, about half of the difference between people's IQs is due to genes.
Thousands of identical twin studies. Raised together, raised apart, they have the same IQ +/- 5 points. When there is a significant deviation, the problem is usually correlated with a lower birth weight in the twin with the lower IQ. A shared placenta can lead to one of the twins getting less nutrition and oxygen. The lighter twin sometimes gets the "used" blood and waste by-products as well.