According to the Hardcore History podcast, the general public had a superstition that executioners were untouchable. As a result, executioner families could only marry other executioner families. After a few generations not only were they socially outcast from society, but also weird in that inbred kind of way.

Another "fun fact":

Charles-Henri Sanson's eldest son Gabriel (1767–1792) had been his assistant and heir apparent from 1790, but he died after slipping off a scaffold as he displayed a severed head to the crowd. With his death, the hereditary obligation fell to the youngest son.

Used to know someone whose father was descended from the last Sanson executioner and whose mother was descended from one of his last victims.

and i thought the Hanson brothers were bad ass