https://news.yale.edu/sites/default/files/styles/horizontal_...
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This subtly dehumanizing idea that certain races are normal while others are irregular is the root of many problems.
The US, however, has had a large black population for many generations now. Over time, many have mixed with other races in the US (as with every race there really) and consequentially you may have people that are 1/4 black genetically but who hold strongly to their identity as a black person. Often this has stemmed from racism i.e. if someone's child is a quarter black, they're seen/treated as black by other races and therefore it's the identity they form for themselves.
Shaun King, a prominent black activist, is an example of someone who on sight many outside the US (and many inside the US, from what I've seen), would absolutely not consider a black man, but genetically he apparently is and it's central to his identity.
Photo: https://cdn.cnn.com/cnnnext/dam/assets/150819225407-black-li...