I don’t have a factual answer, but its interesting we use skin color and not things like eye color or hair color to describe people. Nobody breaks people into groups based on hair color or eye color.
Within the US at least, racial divisions were intentionally built into our culture to prevent any kind of lower class alliance. In Bacons rebellion, poor/enslaved whites and blacks united together to fight the aristocrats. So the aristocrats created something called the virginia codes, designed to give black people less status than white people. The goal was to make white people identify based on race instead of class, and to view blacks as inferiors who they would never ally with. This was in 1705 and now its 2025, and its still alive and well.
However, this is from the US, not globally. My understanding is things like intermarriage between whites and blacks was more common in the 17th century.
So intentional focus on difference in skin color is built into US culture to discourage any alliance between the lower classes against the upper classes. But again, that doesn’t explain why its a global phenomena.
Well, there was schoolteacher Jane Elliott’s famous eye-color exercise in the late 1960s, where she divided her 3rd grade class into two groups based on whether they had brown eyes or blue eyes (they were all white, so she had to use something other than skin color to make the point). Initially, she told them that brown-eyed children were superior. The “superior” children grew arrogant and discriminated against their “inferior” blue-eyed classmates. She then switched the roles, so that the blue-eyed children were the “superior” children and the brown-eyed children were the “inferior” children.
It was quite effective in demonstrating how something insignificant can lead to baseless discrimination.
She took a lot of flak for it at the time. She would probably have been fired and banned for teaching for life today.
Speaking as a used-to-be redhead, i wasn’t even aware that anyone discriminated against red-headed people until i was about 50. It’s really not a thing in the US.
I suspect it is in the UK because it’s a marker of Irish heritage, but I’m not sure.
ETA: the Wikipedia article says there was discrimination against redheads in Germany and Spain because people suspected redheads of being Jewish. I think Ashkenazi Jews have the second highest incidence of red hair after the Irish. But i am Jewish, so if people are discriminating against me because i might be Jewish, I’m going to attribute that to antisemitism, not to anti-red-hair-prejudice.
I recall being late 20s when my undergrad college chum, a white nth generation American from California like myself, married a young English woman he’d met a few years ago while at grad school in England.
Her withering contempt for all things Irish, and her liberal use of “Irish” as a swear / slur word about like a 1960 white American might use the N word was an absolute revelation to me. I was certainly familiar with racial prejudice. But those two groups, English and Irish, were the exact same color and indistinguishable by sight. How could they be discriminatory to each other? Answer: Very easily. That’s how.
Lists of facts about Presidents traditionally included their heritage. Scots-Irish predominated. For years I was puzzled by that designation, because I didn’t see it elsewhere. I knew that many Scots had emigrated to Ireland, but it seemed strange that so many Presidents’ families came from that small area.
Finally, the penny dropped. Scots-Irish was a code word for Protestant. Irish was the code word for Catholic, although it was never needed until Kennedy. All the earlier Presidents had been from “proper” acceptable religions. Sometimes prejudice can be overt and subtle at the same time.
Modern lists have apparently dropped Scots-Irish as a heritage, opting for more specific tags.