What is "evil," medically speaking?
Role-playing games often make rather a big deal out of morality. In Dungeons & Dragons and Pathfinder, we talk about alignment. In the old World of Darkness games, we kept careful track of our humanity and our conscience. Whether you're tracking your reputation as a super-hero or your corruption as a devil, many games encourage or require a player to have some understanding of whether their character is a good or bad person, or at least, whether they appear to be good. In the real world, things are rarely so cut and dry. Much as I wish I could look at the person next to me and see from their aura whether or not they're trustworthy, I can't do that, and I've never had the pleasure of meeting someone who could. In my decade or so of working in the field of mental health in one capacity or another, I've interviewed murderers, rapists, and psychopaths -- some of whom, I have to say, were other doctors -- and only two or three times in my life would I say I'd felt someone was truly deep-down "evil" once I'd gotten to know them a bit, so qualifying evil in absolute terms is something I find... let's say, questionable. From a scientific and medical perspective, no one has ever come up with a satisfying definition for what evil is, but we have come up with some insights into something else interesting: where evil comes from inside of us.
First off, we need to operationalize a couple of terms. In medicine, there's no such thing as "evil" per se. Rather, we talk about antisocial behaviour, as typified by individuals with antisocial personality disorder (ASPD). A person with antisocial personality is classically someone who lacks empathy for others and rarely factor's others' wants, needs, or feelings into their decisions; basically, they have a persistent disregard for the rights of others. The often feel diminished (not necessarily absent) guilt when they hurt someone, and consequently are much more likely to hurt someone else on purpose. People with ASPD are often very impulsive, frequently have a hard time planning for the future, and seem not to learn from being punished. ASPD is a spectrum, not a discrete diagnosis, and someone who has "traits" as opposed to a full-blown disorder could be an excellent friend and a good family member... as long as you stay on their good side. ASPD is distinct from actual "psychopathy," which can be very loosely defined as "the worst cases of antisocial personality." The current bible of psychiatric diagnosis the DSM-IV, doesn't include psychopathy, but it's a validated construct with a growing body of research behind it. In gamer terms, probably every evil character has some degree of antisocial personality traits, but not every character with antisocial traits is evil (although the ones who aren't evil are probably almost all chaotic). In the real world, ASPD is estimated to be found in about 3% of men and 1% of women; in my game worlds, about 3% of all people have evil alignments.
Contrary to what many people seem to believe (or wish for), researchers have yet to isolate a single gene which turns people evil. That being said, a number of genes have been associated either with ASPD or, more broadly, some behaviours linked with ASPD, including impulsivity, criminality, and poor decision-making. Some of the same genes have also been linked with both suicide risk and non-suicidal self-harm, like wrist-cutting. I'm going to list just two of these genes, but there's plenty more reading material out there for the curious.
Probably the best studied gene in this area is the 5HTTLPR gene, which stands for the "serotonin transporter-linked polymorphic region." Serotonin, or 5HT, is a neurotransmitter in the brain which is associated with numerous mental disorders; low levels of serotonin or inefficient use of serotonin in the brain seems to predispose (but not cause) to various forms of depression and anxiety, and the most popular antidepressants on the market today work by increasing serotonin use in the brain (as well as other neurotransmitters, because nothing is ever simple). The 5HTTLPR plays a role, not in the creation or metabolism of serotonin, but in how it gets transported around, and the shorter version of the gene, which is less efficient, does a poorer job of moving serotonin where it needs to be. People with the short form of the gene seem to be more impulsive, more likely to harm others, more likely to harm themselves, and generally more likely to have a hard time dealing with their internal lives. These people also tend to be resistant to many psychiatric medications, which may be one reason why we don't have any drugs that significantly reduce criminality.
The other interesting gene is called MAOA, which is the gene which codes for a protein known as monoamine oxidase A. Monoamine oxidase A is an enzyme that the body uses to break down monoamines, a class of chemicals which includes serotonin and a number of other neurotransmitters. As with the 5HTTLPR, shorter forms of MAOA seem to be less efficient and are associated with increased risk of violence and criminality. What's really interesting about this is that people with the shorter MAOA gene seem to be at greater risk of being violent and criminal primarily if they're abused as children; if they aren't abused, neglected, or otherwise traumatized, they don't seem to be at much higher risk of growing up prone to violence. The gene might therefore be thought of as a vulnerability, but it certainly doesn't cause a child to be evil from the moment of birth. On the other hand, non-traumatized people with the less-efficient have been shown in a lab to have behaviour which might be called "more vengeful"; when they're slighted, they seem to be more likely to react excessively than people with longer forms of the gene. We're probably still decades away from understanding what it really means for someone to have one or both of these genes, and we certainly aren't anywhere near being able to "cure" people of them.
Of course, all of this assumes that you're raised in a culture where evil is a bad thing. If you're raised in a society where slavery or human sacrifice is normal, acceptable, and even morally required, you'll almost certainly grow up to feel the same way regardless of your genetic vulnerabilities. The question of what drives a person to grow up good in a society where evil is the norm is one which modern scientists have yet to effectively tackle.
A little more than four years ago, Dr. Eris Lis, M.D., began writing a series of brilliant and informative posts on RPGs through the eyes of a medical professional, and this is the one that appeared here on March 16, 2013. Lis is a physician, gamer, and author of the Skirmisher Publishing LLC OGL sourcebook Insults & Injuries, which is also available for the Pathfinder RPG system.