If race is a social construct then why can't I get sickle cell anemia, a disease that only affects black people?
You can get it too actually if one of your ancestors had it. It has nothing to do with skin colour. The reason why it's so common in Sub-Sahara Africa is that is
kind of gives them a resistence to Malaria. Thus people with said disease are more likely to survive and reproduce.
^^ There is a 15 point IQ difference between whites and blacks. This is scientifically measurable and has remained constant.
I would like to point out that
most IQ-Tests are created by well educated (most likely) white people. As far as I know in German speaking countries there is an unintentional bias towards people coming from richer families. Since it's not certain how much genes affect IQ, it's safe to assume that people with better education have better results.
Blacks almost never curbed their own peafowlish traits which caused them to devolve.
If you mean with "peafolism" birthrate, then I would like to remind you that birthrates were similar high in western countries centuries back. It has nothing to do with race, but with other factors. Namely job, family and environment. Farmers who need kids to work on the field with an already high infant mortality rate will make sure they have as many children as possible. To ensure they will able to continue even in future genrations.
Even in western countries this trend is there. Rural families have usually more children than families living in urban surroundings. Now does that mean they have lower IQ? Most certainly not. They just happen to have a reason to have (more) kids.