I can't give a cite for the following I read it in either a newspaper or magazine probably 30+ years ago. There was a school district in a small town that for the first time gave IQ tests to all of the children in some younger age group, I think around 7 or 8. The results showed a several point difference between the white kids and the Black kids. The district wrote this off as due to cultural bias in the tests. One…
While it's uplifting story and I can easily believe that higher lead exposure for Black kids over white kids led to lower IQ results in tests in that specific case, there are no school districts in US with no gap between white and Black students. So far there has not been identified single parameter that explains White/Black gap and not for the lack of trying.
Actually there has been, we're just not allowed to apply the components of the explanation to this context, so we continue on a fruitless search in every other direction, spending money, contorting society, and increasingly tearing down achievers in a misguided attempt to bring up the underperformers.
We know of hundreds of genes which are strongly correlated with IQ and directly influence behavior, and we know how those genes are distributed among various demographics. The science itself is not controversial or uncertain. But the logical conclusions are social and career suicide.