Even in the 20th century, there were instances in which African Americans were still considered of a lower class than white Americans, despite the abolition of slavery. Evidently it is not possible to change race, and therefore this is one of the more significant examples of social standing being determined at birth. If American society were to move back to the age in which African Americans were predominantly identified as slaves, it would probably cause mass rioting and society would become unstable. Even if the change were to be accepted, it would be unlikely that many years would pass before there were movements to abolish slavery and reinstate equal rights.
If society were to move back to a feudal age in which socioeconomic standing were still the primary determinant of adult lifestyle, there would be many repercussions. The true extent of love marriage which occurs today would be impossible, as it was severely frowned upon to marry someone outside of your own class. Marriage would again be used as a tool for upward mobility. There would be increased segregation as it was also frowned upon to socialize with those lower, and therefore friendship groups would become more closed. Education for many would also suffer, as it is unlikely that scholarships would be in existence, and many bright young minds may be ignored because of their socioeconomic class. It is hard to imagine a society in which wealth and social standing is the primary determinant of importance, and it is difficult to imagine how society would react to regressing to this