Those who are interested in such things would do better to look at family structure, ethnic traditions, and social conditions. To correlate race and intelligence in the name of science and have the world pay attention to you is no small feat. Even the most sophisticated numerical analysis which showed a correspondence between phlogiston and ether wouldn't get a hearing today. Some probably assert these things as a matter of establishing power. Being a member of a pure race is a quick and simple way to establish one's superiority.
Membership is easy.
You're born into it. Being the right race gives one a right to superiority and justifies inequality. It also justifies racism, since if inferior people are succeeding they must be cheating the truly superior people out of their just inheritance. It also justifies believing things about oneself that have no objective validity. A truly inferior being can justify thinking of himself as superior because of his race membership.