1. The racism tweet
To oppose racism, you don't have to say "races don't exist"- Bad idea to depend on this dubious empirical claim. https://t.co/Wa7I9RN3jd— Steven Pinker (@sapinker) June 25, 2016
Now to the part I do not agree with. As I understand, the term race is not used in any formal biological nomenclature (see here). I do understand that genetic clusters within humanity are a reality and that these have practical implications. A first remark is that statistical clustering can lead to different clusters depending on how clustering methods are used and on what attributes one clusters. A second remark is that even one study which found genetic clusters which roughly correspond to major geographical regions (Rosenberg et al. (2005) cannot be seen as a justification for the existence of human races.
As lead author of the study Rosenberg himself said: "The study's overall results confirmed that genetic difference within populations is between 93 and 95%. Only 5% of genetic variation is found between groups" and that the "findings “should not be taken as evidence of our support of any particular concept of biological race" (...).
The term 'race' as it is known by non-scientists refers to a vague concept which, I think, does far more harm than good. For example, people use the term race to refer to as diverse things as physiological features, geographical factor, religious factors, nationality, cultural habits, etc.
By implying that races are an empirical reality (which I don't buy), I am afraid Pinker reinforces vague common sense notions of race such as: there is a white race and a black race, there is such a thing as the Jewish race, Mexicans are a race, Muslims are a race. By accepting such vague terms you allow the term to be used for terrible purposes.
Pinker accuses his opponents of trying to advance their purpose (let's say egalitarianism) by relying on an argument which is not grounded in reality (the idea that races do not exist). Ironically, he may be the one who hangs on to a concept which is dubious. By the way, the onus is not on the skeptic to prove that races do not exist. The onus is on the scholars claiming races do exist to clearly define the concept and to provide empirical evidence of their existence. Even if they would do that, they'd better avoid the term race in order to prevent that people would see this scientific evidence as support of their mistaken intuitive and bigoted notions of race.
2. The tweet on sex differences
Pretending the sexes are identical doesn't help women: Unisex parental leave boosts male profs at women's expense. https://t.co/BZtj75R1K4— Steven Pinker (@sapinker) June 26, 2016
Having said this, I'd like to point out two things. First, the fact that biological tendencies exist does not mean we should always accept them. Human civilization, in part, is a matter of creating institutions and behavioral habits which counter biological tendencies. As I mentioned here, inborn biological tendencies are shaped into us by evolution to serve our genes. Our individual and collective interests overlap only partly with these interests. It takes careful reflection to determine when to resist biological tendencies and when to go along with them.
Second, we must be careful about how we frame the issue of sex differences. The frame implied in the tweet is that the question is either whether sexes are identical or not. As I said, I agree that sexes are not identical. However, this frame masks a large part of reality which a different type of frame may reveal. Instead of asking "are sexes identical or not?", we may ask "in what respects are sexes equal and in what aspects are they identical?". Answering this question reveals that men and women are different in certain specific ways but equal in many other ways, I think in most ways.
Primitive societies have both viewed and treated men and women much more unequally than advanced societies. I would not be surprised that still more advanced societies will view and treat men and women even more equally and appreciate our commonalities even more.