Current definitions of narcissistic personality disorder stick closely to those outlined in the 1960s. A person with the disorder, let's call him a "raging narcissist" to distinguish him from the garden-variety subclinical species that we all encounter every day, has a deluded sense of self-importance and a voracious appetite for praise. Deep down, of course, a raging narcissist is completely uncertain of his awesomeness and is, as a result, extremely edgy about criticism. An important addition to the definition is the lack of empathy that is now considered highly characteristic of the disorder [source: Mayo Clinic].
What makes a narcissist? Nobody really knows. Maybe too much parental coddling, maybe too little. Maybe a fatal combination in which parents insist a child is special but withhold attention and subject him to excessive criticism? But behavioral pathologies are too complex to untangle at the root. Genetics and environment (i.e., nature and nurture) create such an intricate dance of causation that nobody even pretends to understand what it takes to mold a full-blown narcissist.
As for the causes of narcissism itself, research in the field of evolutionary psychology might hint at why many among us seem to really like ourselves. A 2004 study used computer modeling techniques to look at facial resemblances between mates. The researchers found that when we get together to make babies, it seems we're not just looking for our "soul mates." Theorists used to assume that mating was a random business and there was no accounting for taste. "The heart wants what it wants," to put it technically. But no, what we're really after are individuals who look like us. Researchers call this "assortative mating," which means "self seeking like." Apparently this behavior is quite common across species.
There are, it seems, good evolutionary reasons for this assortative mating strategy, chief among them being something called genetic stability. Luckily there are other forces at work to prevent too much similarity (i.e., inbreeding) with the result that we tend to mate with a goldilocks partner — one who looks like us, but not too much [source: Alvarez and Jaffe].
So it could be that narcissism is a useful inborn trait that guides our mate selection; however, it curdles into unpleasant behavior when we fail to develop the ability to look for our reflection in the faces of others, rather than a mirror.
Seeing ourselves in others is, of course, also a key to empathy, a trait notably lacking in many, if not most, narcissists. Scientists in Berlin used this key factor to look at the brain chemistry of people with narcissistic personality disorder. Magnetic resonance imaging (MRI) revealed that the cerebral cortexes of their test subjects were significantly thinner than normal [source: Charité]. The cerebral cortex, which is the external nerve cell layer of the brain, is where we foster empathy.
So that's it, isn't it? Your brain is your brain, and if you've got a skinny cortex, there's nothing to be done but stare in the mirror and marvel at what you see, regardless of the consequences. Well, maybe not.