In psychopaths, the brain physiology and functions responsible for understanding the feelings of others is impaired. A lack of communication among the parts of the brain that generate emotions results in the psychopath’s insensitivity. Other complicating factors include hormones and other biological functions.
Two recommended books that discuss psychopathic genetics:
The Psychopath Whisperer, the Science of Those Without Conscience, by Dr. Kent A. Kiehl.