Actually that's not what I said it all. The concept on in/out-group can be completely arbitrary, e.g. assigned by flipping a coin. The principle still holds.

That said, since you bring it up, the history of humanity is largely the history of war between different ethnic groups. So yes, hating people based on ethnicity has been the norm for most of human history. It's still largely the case in varying degrees in different parts of the world.

> since you bring it up

Nice try.

> So yes, hating people based on ethnicity has been the norm for most of human history

There you go.

And yet history proves you wrong, starting with the roman empire and the middle ages.

> Nice try.

You did. Right here: "e.g. ethnicity". That's a quote from you. I mentioned in-group, you added ethnicity.

> There you go. And yet history proves you wrong, starting with the roman empire and the middle ages.

At this point I wonder if you are trolling. The roman empire waged war on pretty much every single other ethnic group around them and enslaved them. Which the other ethnic groups around them were also trying to do at the time. Then we have the middle ages with the crusades and such... etc etc etc