V. S. Ramachandran is a neurologist and an author. If you're interested in the mysteries of the human brain I highly recommend you read any of his books. In a lecture from 2006* he talks about "mirror neurons". Scientists have found motor neurons fire in a monkey's brain when the monkey reaches for a peanut. Interestingly, a subset of these neurons, called mirror neurons, fire when the monkey watches another monkey reach for a peanut. Mirror neurons in humans work the same way. The motor neurons fire when we poke a person with a needle, but a subset of these neurons, the mirror neurons, fire when we witness someone else being poked with a needle. These mirror neurons appear to be our way of empathizing with another. They are the mechanism by which we put ourselves in someone else's shoes and are the basis of our empathy. As such, they may be the basis for human ethics.
Ramachandran notes that autistic children appear to suffer from mirror neuron dysfunction, resulting in a lack of empathy and an inability to relate to others. A separate article about mirror neurons notes that psychopaths and sociopaths have impaired functioning of the mirror neurons. Psychologists have known for some time that people with antisocial personality disorder (the clinical term for sociopaths and psychopaths) feel no empathy and have no regard for the rights of others. Narcissists too share these traits. Though they both see themselves as human, they fail to see the humanity in other people. They regard people in their lives as objects to be manipulated to get what they want, not as human beings. People with antisocial and narcissistic personality disorders are unable to relate to other people as being like themselves. People with these personality disorders feel no empathy for other people, they cannot identify with other people and they feel no remorse when they cause harm to others.
We knew that a horribly neglectful or abusive childhood could result in antisocial or narcissistic personality disorder, but we didn't know the mechanism. It is now believed that such a childhood can result in these neurons not being used, so they fail to develop normally. This failure to develop in childhood results in an adult with dysfunctional mirror neurons and the resulting antisocial or narcissistic traits. If these neurons can be rendered dysfunctional by lack of use, perhaps we can develop interventions which use them, restoring their functionality and healing people with these personality disorders.
It's an interesting theory.
* Ramachandran's part of the lecture begins about 39 minutes into the presentation.