Why do light-skinned blacks look down on dark-skinned blacks or think they are better than the dark-skinned?

The concept of racism, in it's broadest sense, goes way back before the slave trades and scientific racism of white Europe and America. Dark-skinned peoples from various cultures going back to ancient times have engaged in racism among their own ethnic groups, dividing cl systems based on skin variation (skin colour variations exist within any single ethnic group.) It's thought that lighter skin has been ociated with higher social cl because those of nobility probably spent far less time outdoors in the sun and therefore did not develop a darker skin tone compared to slaves and those of the lower cles who spent much more time working outdoors exposed to the sunlight. This tradition was compounded by the extreme form of racism developed by white Europeans and Americans which eventually lead to scientific dogmas revolving around the concept. Although racism is no longer held within science, the racist tradition of ociating lighter skin tone with higher social cl still persists across many cultures.