We're really talking past each other; not sure why. Where am I getting either the facts or your position wrong?
1. Assume we first measure “race” just by using self-reported data. Would you agree: race will vary strongly with differences in physical characteristics, ...
If you look in the mirror and you see a black person you'd think you would self identify as a black.
I spent some time in the Dominican Republic. People who look like classic blacks per an American eye consider themselves Hispanic, including talking about blacks as a different race. The reason is, they look down on the Haitians as an inferior 'race'.
and with many psychological and behavioral variables? In some cases, race will be a meaningful variable in epidemiology?
I'm not convinced there's a lot of data suggesting behavioral & psychological correlations to race when one speaks of 'nature'. Without adding in 'nurture' which is going to vary even more across biological lines, I don't agree.
I've already addressed the epidemiology question. Self identified race has some screening value but it's subject to a lot of false negatives and false positives.
2. Race is most definitely a social construct.
Agree
The hypothesis here is that it might also have a biological / genetic basis. In other words, perhaps “race” is a useful genetic distinction.
Not very useful at all in light of the much better refined genome sequencing.
You keep conflating two things.
The fact traditional racial features are genetically based. They are.
And the fact when one is looking at genomes one would see a good reason biologically to group those specific racial features together. There has been no good reason cited here.
Instead of going on in this mulberry bush circle, clear your head and consider this:
Say you only had the genome sequencing and you knew a little about populations but nothing whatsoever about what individuals looked like, and you made groupings of individuals based on the genomic data.
You imagine the groupings would correlate with races because you read that a researcher looking backward from appearance found correlating clusters of genetic sequences.
However, a biologist looking forward would start with the last common ancestors and work forward to create groups. And when you do that you don't get nearly as close in grouping populations. That is called following the evidence.
If you start with the conclusion and work backward, you end up making the evidence fit by ignoring all the evidence that contradicts your conclusion such as the non-correlating to race but correlating to geographic clustering of blood groups.
That's why in the scientific process one follows the evidence and one doesn't look to make the evidence fit the conclusion.
Now if you want to look at the social construct conclusion, you do what people did before we had all this genome mapping, you follow the evidence which we collected using the social construct of race.
I think that answers 3 & 4 as well.
p.s. you did use the word “traits” plural in your blood group analysis post.
Which traits correlated with blood groups did you have in mind?
There are dozens of specific proteins and genetic differences that create blood groupings. I thought you could see that from the link I cited.