We’re talking generic ‘selfishness’ vs. narcissistic personality disorder, as defined by the DSM-IV, correct? (just to differentiate between this and the colloquial usage of ‘narcissistic’)
I would say they are distinctly different; much like one can be very sad without having Major Depression, one can be selfish without having a personality disorder. One can be selfish without being narcissistic. In fact, selfishness seems to be a minor and somewhat indirect feature of NPD:
A pervasive pattern of grandiosity (in fantasy or behavior), need for admiration, and lack of empathy…[characterize NPD].
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Narcissistic Personality Disorder (NPD) is a pattern of self-centered or egotistical behavior that shows up in thinking and behavior in a lot of different situations and activities. People with NPD won’t (or can’t) change their behavior even when it causes problems at work or when other people complain about the way they act, or when their behavior causes a lot of emotional distress to others (or themselves? none of my narcissists ever admit to being distressed by their own behavior—they always blame other people for any problems). This pattern of self-centered or egotistical behavior is not caused by current drug or alcohol use, head injury, acute psychotic episodes, or any other illness, but has been going on steadily at least since adolescence or early adulthood.
NPD interferes with people’s functioning in their occupations and in their relationships…
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Grandiosity is the hallmark of narcissism. So what is grandiose?
The simplest everyday way that narcissists show their exaggerated sense of self-importance is by talking about family, work, life in general as if there is nobody else in the picture. Whatever they may be doing, in their own view, they are the star, and they give the impression that they are bearing heroic responsibility for their family or department or company, that they have to take care of everything because their spouses or co-workers are undependable, uncooperative, or otherwise unfit. They ignore or denigrate the abilities and contributions of others and complain that they receive no help at all; they may inspire your sympathy or admiration for their stoicism in the face of hardship or unstinting self-sacrifice for the good of (undeserving) others. (via)