So we have talked a lot about the narcissist but how do you tell the difference between a really selfish person and a narcissist?? In a narcissist there just is no one else unless they are there to gratify the narcissists grandiose view of herself or himself. Others are just tools. From biological studies it has been determined that certain areas of the narcissistic brain just don’t fire like an ordinary person. I will not say normal because there really is no such thing we can say the average person. They are the center of the universe. People are gathered and discarded with ease. You are no more to them than a new car or other possession that signals their superiority to others. As long as you tow the line or they don’t get bored you are allowed their presence. However once a new object is seen and they are bored you are discarded with no more care than one discards used toilet paper. As “normal” or average people we don’t understand this.
Selfish people however in my estimation are different they have never progressed from being a childish or juvenile brain. All children are born selfish they learn empathy from others. This then is a psychological not a biological condition and could be overcome.
Therefore I believe that these two conditions overlap the selfish person who also has a biological component can become a narcissist, sociopath or psychopath as these are just degrees of the same condition. Nature and nurture. A narcisssist brought up in a caring home probably will still be a narcissist to some degree. A person that has a normal brain will not become selfish without the nurturing of a selfish nature.
Here is more information gleaned from the internet. Either way a relationship with a selfish or narcissistic person will be doomed to fail unless the object is attracted and wishes to be a subservient. Even then you must bear in mind that you may just be the flavor of the week.
Pathological narcissism traits:
Pathological narcissism is a life-long pattern of traits and behaviors which signify infatuation and obsession with one’s self to the exclusion of all others and the egotistic and ruthless pursuit of one’s gratification, dominance and ambition.
As distinct from healthy narcissism which we all possess, pathological narcissism is maladaptive, rigid, persisting, and causes significant distress, and functional impairment.
Pathological narcissism was first described in detail by Freud in his essay “On Narcissism” (1915). Other major contributors to the study of narcissism are: Melanie Klein, Karen Horney, Franz Kohut, Otto Kernberg, Theodore Millon, Elsa Roningstam, Gunderson, and Robert Hare.
What is Narcissistic Personality Disorder (NPD)?
Narcissistic Personality Disorder (NPD) (formerly known as megalomania or, colloquially, as egotism) is a form of pathological narcissism. It is a Cluster B (dramatic, emotional, or erratic) personality disorder. Other Cluster B personality disorders are the Borderline Personality Disorder (BPD), the Antisocial Personality Disorder (APD), and the Histrionic Personality Disorder (HPD). The Narcissistic Personality Disorder (NPD) first appeared as a mental health diagnosis in the DSM III-TR (Diagnostic and Statistical Manual) in 1980.
The ICD-10, the International Classification of Diseases, published by the World Health Organisation in Geneva  regards the Narcissistic Personality Disorder (NPD) as “a personality disorder that fits none of the specific rubrics”. It relegates it to the category “Other Specific Personality Disorders” together with the eccentric, “haltlose”, immature, passive-aggressive, and psychoneurotic personality disorders and types.
The American Psychiatric Association, based in Washington D.C., USA, publishes the Diagnostic and Statistical Manual of Mental Disorders, fourth edition, Text Revision (DSM-IV-TR)  and fifth edition , where it provides the diagnostic criteria for Narcissistic Personality Disorder (301.81, p. 669).
Both editions of the DSM define Narcissistic Personality Disorder (NPD) as “a pervasive pattern of grandiosity (in fantasy or behaviour), need for admiration, and lack of empathy, beginning by early adulthood and present in various contexts”, such as family life and work.
The DSM specifies nine diagnostic criteria. Five (or more) of these criteria must be met for a diagnosis of Narcissistic Personality Disorder (NPD) to be rendered.
[In the text below, I have proposed modifications to the language of these criteria to incorporate current knowledge about this disorder. My modifications appear in bold italics.]
[My amendments do not constitute a part of the text of the DSM, nor is the American Psychiatric Association (APA) associated with them in any way.]
[Click here to download a bibliography of the studies and research regarding the Narcissistic Personality Disorder (NPD) on which I based my proposed revisions.]
Proposed Amended Criteria for the Narcissistic Personality Disorder
Feels grandiose and self-important (e.g., exaggerates accomplishments, talents, skills, contacts, and personality traits to the point of lying, demands to be recognised as superior without commensurate achievements);
Is obsessed with fantasies of unlimited success, fame, fearsome power or omnipotence, unequalled brilliance (the cerebral narcissist), bodily beauty or sexual performance (the somatic narcissist), or ideal, everlasting, all-conquering love or passion;
Firmly convinced that he or she is unique and, being special, can only be understood by, should only be treated by, or associate with, other special or unique, or high-status people (or institutions);
Requires excessive admiration, adulation, attention and affirmation – or, failing that, wishes to be feared and to be notorious (Narcissistic Supply);
Feels entitled. Demands automatic and full compliance with his or her unreasonable expectations for special and favourable priority treatment;
Is “interpersonally exploitative”, i.e., uses others to achieve his or her own ends;
Devoid of empathy. Is unable or unwilling to identify with, acknowledge, or accept the feelings, needs, preferences, priorities, and choices of others;
Constantly envious of others and seeks to hurt or destroy the objects of his or her frustration. Suffers from persecutory (paranoid) delusions as he or she believes that they feel the same about him or her and are likely to act similarly;
Behaves arrogantly and haughtily. Feels superior, omnipotent, omniscient, invincible, immune, “above the law”, and omnipresent (magical thinking). Rages when frustrated, contradicted, or confronted by people he or she considers inferior to him or her and unworthy.