Wait. Is it racial or is it cultural? There's a difference there.

Do you believe it's possible to have cultural homogeneity without having racial homogeneity?

Btw I think you're wrong: people can be extremely xenophobic with the folks from the next village over who are their cousins thrice removed. It's nothing to do with race or culture, just who's in the in-group and who's in the out-group, and what those are depends on a myriad of factors that "race" and "culture" are only convenient, but inaccurate, proxies for.

Another example: Greek football fans from Athens will literally murder their counterparts from Thessaloniki, and they're all just as Greek as each other [1]. You just have to draw a line in the sand and people on either side of it will start throwing stones at each other.

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[1] Culturally of course. We're not a "race". Tiny little rant but with Nolan's Odyssey, there were the usual complains about Helen being played by a black woman etc. and people all over the internet were having fights about what real Greeks look like. Oh they have fair hair and light skin. No they have dark hair and olive skin. And so on. The truth is that most Greek families are like mine: one parent is fair-haired and light skinned (my dad: blond hair, green eyes, goes red in the sun; his mom: blond hair, blue eyes, skin like yogurt) and the other is dark-haired and dark skinned (my mom: brown hair, brown eyes, goes brown in the sun; her mom: black hair, black eyes, never sat in the sun). But stereotypes persist so me and one of my friends were asked how come we're both fair-haired with light-coloured eyes when true Greeks are brown and at the same time a friend recommended to me a concealer that was three tones darker than my actual colour even though I was standing right there in front of her eyes with my pasty-white skin. Race is not what it's made out to be. End rant.

I think cultural is probably the thing that has benefits, but they are so intertwined that they're essentially the same or impossible to extricate. Anyone who has visited Japan can understand that there is some benefit to racial/cultural homogeny. It doesn't really matter which of the two is the important part when they cannot be extricated.