Apparently the McCoy family and their descendants have a genetic disease not uncommon to the area at the time called Von Hippel-Lindau Disease, which causes the formation of many non-cancerous tumors and cysts in the cerebellum, retinas, inner ear and spine. Aside from the physical consequences of this, It can put pressure on various cranial glands and cause hormonal and emotional imbalances that can make a person quick to fear and anger. I would link an article but when I searched “McCoy Von Hippel Lindau” the bylines were so eyeroll-inducingly sensationalistic that I couldn’t bring myself to include them