Average IQ scores are increasing across birth cohorts, such that Americans experienced an 18-point gain in average IQ from 1948 to 2002. And the most decisive and permanent environmental intervention that an individual can experience, adoption from a poor family into a better-off one, is associated with IQ gains of 12 to 18 points.If this were true, then everyone would be in favor of those policies that increase IQ. But Americans are getting dumber, not smarter. And adoption is not helping.
Consider this new adoption study on the heritibility of IQ:
Heritability of IQ was estimated to be 0.42 [95% CI 0.21, 0.64]See also discusssion here.
Proportion of variance in IQ attributable to environmentally mediated effects of parental IQs was estimated at .01 [95% CI 0.00, 0.02]
That is, smart parents have smart kids. You might think that the cause is White privilege, or having enclopedias at home, or sending kids to good schools, or better nutrition, or some combination. But nobody has found any parental strategies that work, other than having better genes to start with.
If this were the only study, I would not believe it. But dozens of studies have been showing this for decades. It is probably the most well-verified theory in the entire field of psychology. Psychology is filled with untestable nonsense, such s Freud, and studies that failed replication, such as what fills the textbooks. But the IQ studies have been well-replicated.
Nobody likes these results. They suggest genetic determinism, and no one wants to be a pre-programmed robot. Furthermore, they suggest that all the things parents do to improve their kids intelligence are wasted.
Leftists especially hate these results because all their programs to close IQ gaps are doomed to fail.
There are similar results for bahavior traits that may even be more important than IQ.
This is part of why the Left has shiftwd from "equality" to "equity". They know that equality of opportunity will never lead to equality of outcomes.