Many of the studies showing this have some serious flaws, because of the types of people they tended to use. The twins used in the studies where disproportionately from higher income families. You don't find many low SES families who adopt, so for this and other reasons it's very difficult to find twins raised apart in low SES families.

The summary is that it's pretty clear that among higher SES families IQ is more than 50% genetic (but environment still plays a very large role). However, many studies indicate that among lower SES families, environment is the dominant factor.

There's still conflicting evidence here though, because there are some studies that indicate that SES has little impact on IQ heritability. The argument is that SES has a much greater on the heritability of IQ in children than on that of adults.