They determined a long time that they were right in their opinion. Their belief, knowledge, philosophy... we're not sure as it was a log time ago, but they determined that they were right and any questioning of that Rightness was antagonistic toward their culture. So, they decided to lobotomize those that questioned the Rightness. It wasn't a full lobotomy, just the free thinking portions. Eventually, they were all lobotomized, down through generations, and every child went through the ritual of having the free thinking portion of their brain removed and The Rightness shoved into it.