Hypothesize that the ability to get humans just to follow orders and not ask questions, and to be content and even enjoy that role, is a result of old-school genetic engineering, i.e., selective breeding. The hypothesized mechanism is that slavery has been so incredibly prevalent throughout human history that there exist likely many people who are not just descendants of slaves, but descendants of thousands of generations of slaves who underwent artificial Darwinian selection such that those who were not subservient were executed and not allowed to procreate.
This would mean there exists a "subservience" or "docility" gene or genes, which can be measured in people. Has it already been discovered but knowledge kept quiet? What other behavior characteristics does it cause?
Inspired by the horrifying politically incorrect belief espoused during the era of American slavery that the African was born naturally subservient. And we seem to have bred subservience into some of our pets, most notably dogs.
Science fiction often depicts an alien "slave race".
This does provide a possible scientific explanation of why Native Americans were unsuitable for being made into slaves, forcing the alternative of very expensive importation from Africa. Was slavery prevalent in those cultures?
Does a gene explain why some people have an easier or harder time learning to follow in partner dancing?
Is the subservience allele being selected for or against in modern culture? Should it be?