It one of the most impactful studies on how lead affects intellectual development, the researcher (who is white) restricted the study to white people only because he assumed (correctly) his results would be ignored if he included African Americans who are disproportionately affected by lead poisoning.

It's been pretty well accepted for decades that this is the case:

>A highly significant association was found between lead exposure and children's IQ (P < 0.001)... There was no evidence that the effect was limited to disadvantaged children and there was a suggestion of the opposite.

https://pubmed.ncbi.nlm.nih.gov/8162884/

That's a 1994 meta-study that turned up in 30 seconds of googling, and I'm sure more recent evidence also exists.