At the risk of sending this way off the rails, notice that Think Hard said "most" not all.
A tiny number of people are born with brains (for lack of a better word) that are faulty in structure to the point that they are unable to feel empathy *or* to tell right from wrong. As children they display anti-social traits from the start and without any form of positive intervention those ant-social traits bloom into psychopathy in adulthood, even if their childhood environment was "normal". Some studies show that positive interventions in this subgroup can encourage these children to mature into adults without psychopathic behaviors. In other seemingly more common cases, the genetic markers for psychopathy are there, and based on environmental factors carriers either do or don't develop into psychopaths. It seems most studies agree that there isn't really a nature/nurture divide; studies now are trying to determine how and why the expression of these genes is suppressed in people born with a predisposition to anti-social behavior.
Here's one paper from 2004 that cites studies of twins. If anyone is interested but doesn't have the time to read the full study, the synopsis is a decent overview.
http://www.ucl.ac.uk/educational-psychology/resources/Viding_2004_NYAS.pdf
I know there's been some other studies in the last 10 years using brain MRIs and documenting where different regions of the brain are active in psychopaths versus control groups.
Here's one that shows distinct frontal lobe differences:
http://www.med.wisc.edu/news-events...w-differences-in-structure-and-function/32979
But people who don't become psychopaths can have these abnormal MRIs as well, so there's more at play. For example, "The Neuroscientist Who Discovered He-Was a Psychopath":
http://www.smithsonianmag.com/scien...covered-he-was-a-psychopath-180947814/?no-ist