It is an article of faith of our current civic pseudo-religion that there are no differences in intelligence between ethnicities or sexes. To suggest otherwise is blasphemy, even if you are supported by evidence. We should go wherever the evidence leads. Our belief that all men are legally and morally equal does not mean we have to pretend that they are physically and mentally equal if they are in fact not.
All that said, I don’t think the evidence actually supports Dr. Watson. There was a study—I forget where to find it at the moment—comparing IQs of members of the same ethnic groups who were raised in different parts of the world. Compare the average IQs of West Africans in West Africa vs, the children of West African immigrants in the US, and compare those to the descendants of West African slaves. You think you find that the average native West Africans have IQs well below 100, the average descendants of West African slaves have IQs somewhat below 100, but the average children of West African immigrants to the US have IQs well OVER 100. Yet however you define “race,” they are certainly part of the same race.
Likewise, the study showed that while Ashkenazi Jews and East Asians in the US have IQs well above average, East Asians in East Asia and Ashkenazi Jews in Israel have IQs in the 90s.
The difference seems to be being a member of a population that culturally values education. West African immigrants bring some undesirable cultural traits with them, but they tend to stay married and push their kids to do well in school so they can be doctors and lawyers, just like Ashkenazi Jews and East Asians in the US, and they achieve similar results. The descendants of slaves don’t share that culture, unfortunately, with predictable results there as well.
The question is "how much"? It appears that genetics is above 50% of the mix.
Genetics probably determines maximum IQ for an individual, and environment determines how close they come to reaching that level.
Keep in mind that the self selected group of people who CHOOSE to immigrate is not likely to be a random sampling of the people from where they come from.
In some cases, they will be people who could not make it, and desire to sponge off our welfare system. In other cases, they will be people of high ability who want an opportunity to succeed.