I was reminded of Chapter 6 of Outliers by Malcolm Gladwell. The chapter is called "Harlan, Kentucky," and discusses the feuds that took place there. He makes an attempt to blame it on genetic heritability, tracing it back to Scotland and forebears who herded sheep and practiced honor culture, with mention of a psych study showing Southern students responded differently to insults. Higher testosterone levels? I can't remember. I think it's unsettled that genetics plays a role, but why not?