In the context of the article, these two quotes are right next to each other. The author is saying this. He's trying to paint cultural homogeneity with the parent country, which is reasonable with first and second generation immigrants, but it's a sloppy way to exclude blacks from the American scores.

Let's be very clear here with another quote:

So similar to my comparison of GDP levels, let us compare Americans with European ancestry (about 65% of the U.S population, and not some sort of elite) ... We remove Asians, Mexicans, African-Americans and other countries that are best compared to their home nations.

The author is removing all blacks as they are 'best compared to their home nation' as he has defined them as 'immigrants'. (he's also exluding hispanics, apparently, despite a lot of them already living on land that was acquired by the US in the 19th century - how these are 'immigrants' is beyond me)

The thing is, blacks in the US don't live like their West African forebears, nor raise their kids like West Africans. They live like Americans and raise their kids like Americans. They are Americans and have been for multiple generations. To exclude them on 'American' scores is pretty racist.

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Also, how can you really make a representative argument when you cut out one third of your subject pool? If the system only 'works' if you exclude them, then that means that the system is not working so well for that particular country.