Right now I'm reading a nonfiction book, SLAVERY BY ANOTHER NAME: THE RE-ENSLAVEMENT OF BLACK AMERICANS FROM THE CIVIL WAR TO WORLD WAR II, by Douglas A. Blackmon. I had no idea this appalling chapter of our history existed! It's about the system of convict leasing widespread in the American South from the 1870s all the way to the early 1940s. Poor, mostly illiterate people, overwhelmingly black men, were arrested on flimsy charges, sentenced to fines they couldn't possibly pay, and essentially sold to mines, factories, or farms to work off the "debt." In a way the system was worse than pre-Civil War chattel slavery, because these "employers" had no financial interest in giving the workers proper food or medical care. If an employer needed cheap labor and had friendly connections with a local sheriff or magistrate, getting a supply of convicted "debtors" was easy. Often no specific charge was even recorded, and many of the "crimes" that were cited consisted of vague offenses such as vagrancy, abusive language, or leaving a job without permission.
This account of institutionalized abuse highlights at least two socio-political facts relevant to constructing imaginary cultures: (1) Ingrained biases hang on stubbornly, and it may require society-wide changes to shake these attitudes loose. For many decades after the Civil War, large numbers of southern white people sincerely believed the welfare of their region depended on keeping black citizens "in their place" and furthermore maintained that the black population (except for a handful of troublemakers) was "contented" with the status quo. (2) Oppression takes different forms, and when knocked down in one guise, it can easily reappear in another if those institutional changes aren't made. In speculative fiction, we can imagine many varieties of social inequity, disguised as well as overt. Suzette Haden Elgin’s short story “We Have Always Spoken Panglish” (it’s online; just google the title) portrays an alien society in which the ostensibly “contented” ethnic underclass can protest their status in only one way, by keeping their native language a closely guarded secret. To complicate matters, in SF different intelligent peoples may live together on the same world or space station, raising the question of whether their differences really do justify some kind of unequal treatment. Suppose an aquatic species and a land-dwelling species, for instance, occupy the same planet, coming into frequent contact at the shoreline? Obviously it wouldn't be fair or even sensible to treat these two kinds of creatures exactly alike. What would constitute fair, equal-but-not identical treatment? Suppose, on Earth, dogs became intelligent? Dogs, as far as we can tell from interacting with them in real life, are pack animals who enjoy obeying a leader and feel insecure without one. How would that facet of canine character affect our treatment of sapient dogs? (Cats, on the other hand, if they attained human intelligence, would of course be capable of ruling the world, except that they wouldn't want to bother.) In Cordwainer Smith’s classic “Ballad of Lost C’Mell,” the Underpeople, genetically engineered from animals, have to fight to get recognized as full citizens.
You can read in depth about Blackmon's research on the legal enslavement of convicts at: