The possibility that violence is inherited through a “murder gene” appears throughout the text. After hearing Andy’s family history, Laurie is the first person to alight upon this possibility. As the Barbers prepare for the trial, it becomes clear that the murder gene cuts two ways: Logiudice might cite it as a possible cause for Jacob’s violence, but it could also be an excuse for violence. Jonathan therefore prepares a defense to mitigate Jacob’s crime: if one inherits the irresistible urge to murder, is their crime completely their fault?

Later in the text, we learn from Dr. Vogel that Jacob, Andy, and Billy all have the MAOA Knockout gene associated with male violence. At the end of the novel, it is understood that the Barber family has a violent history, and it is strongly suggested that Jacob is a murderer. It is left for the reader to decide whether genetics play a role in his crimes and whether Andy himself ever displays evidence of his violent lineage.

It is broadly culturally understood that children who hurt animals lack empathy and may hurt other humans later in life. Derek reveals that Jacob once found a stray dog and later stated he had buried it.