The Bell Curve -
Exhaustive research indicates that no matter what is done, African Americans, ON AVERAGE, are less intelligent, AS MEASURED BY INTELLIGENCE TESTS, (1 standard deviation from the mean), then other groups measured. You can control for income, education of the parents, whatever, and the results hold. This implies no social program will alleviate this difference.
Not everyone is 6'6".
Many liberal individuals downplay the book. It is very long (845 pages) and has a lot of statistics. I doubt few have read it in its entirety.
Here is one objection. From Scientific America -
https://blogs.scientificamerican.com/voices/the-real-problem-with-charles-murray-and-the-bell-curve/
“The Bell Curve” endorses prejudice by virtue of what it does not say. Nowhere does the book address
why it investigates racial differences in IQ." If you ever wanted to know what gobbly gook is, this objection is an example. 845 pages of well researched information and the author of the critic says it is all no good because the authors - Herrnstein and Murray - did not state why they wrote the book?
The book does not explain why there is a difference in the IQ on average of Africa Americans and other groups.
There is a reason
Out of Africa. Human life emerged in Africa. Some groups were enslaved and harassed (would become European and Asian) by the dominant group in Africa (some who would become Africa American). Some of those groups that were harassed left because of abuse or other reason. Others, some who would become African Americans, stayed in Africa.
Those that left Africa and latter became groups other than African Americans encountered more severe environments. Via random mutation and natural selection of the different environments, these groups developed different skills. Some of these skills had to do with long range planning and ability to adapt quicker to changing environments.
Skinny - In the beginning some who would become African Americans were the dominant group. Suppressed group who left Africa developed different skills/genetics. As the world moved beyond Africa the skills possessed by those who stayed in Africa where not as beneficial. As the world moved beyond Africa those that had moved earlier were better suited to take advantage of the changing environment.