Why is it that all narcissists have so many personality traits in common?
Because they are narcissists.
To be a Narcissist in the strictest sense, one would have to fit five of the eight criteria for having the Narcissistic personality disorder. I believe this is explained in depth in the very first FAQ "What is a Narcissist?"
Even then some of their quirks and nuances are chillingly similar. The way they look or don't look at you, The facial expressions. The phrases they use. Mannerisms, etc. Even the way they dress.
What is odd to note is that anyone suffering from any personality disorder has so many traits in common with others suffering the same disorder. Yet any of one "healthy" individual has traits unique from others.
Yes and no. A teenager may exhibit traits of Narcissistic Personality Disorder, BUT a teenager's personality is still DEVELOPING. But just as there are teenage psychopaths who murder, there are teenage narcissists. Many teenagers exhibit narcissistic traits, but this can actually be a normal and healthy phase of growing up. After all, if they don't believe that they can be better, they might collapse into a "loser mentality." The difference shows when they start to…
Most of the time, extreme narcissists would not even be aware of having acted badly; it is not that they don't remember acting badly. From their point of view they simply don't act badly. But narcissistic traits, like all other personality traits, come in varying degrees. Many people with narcissistic characteristics will act badly from time to time, and be able to remember and acknowledge it, even if it costs them some effort.
Borderline personality disorder is a specific personality disorder. A personality disorder is a set of traits and long held, often inflexible, beliefs which tends to interfere with "normal" functioning in society. There are different types of personality disorders. Borderline personality disorder is one of many specific classifications of personality disorders.
There are many traits of schizoid. There are covert and overt traits. Covert are non-obvious traits, and so you have to rely more on the unshared feelings of the person. Overt traits are obvious, prefers to be alone, introversion,narcissity,superiorness,self-sufficiency,loneliness,loss of affect,depersonalization,and regression.
Many personality traits and disorders are hereditary as are many diseases and afflictions. The parental lineage of an animal contributes to the personality of said animal. The parents of an animal are also factors based on what the young animal observes from those parents during the developmental period of an animal.
The addictive personality was described as escapist, impulsive, dependent, devious, manipulative, and self-centered; they are also many times sensation seekers that like a rush of adrenalin, and possibly drugs . Many doctors now believe that these character traits develop in addicts as a result of the addiction, rather than the traits being a cause.
Hans Eysenck and the many followers after his death are not taking into account modern research findings. Wikipedia describes many contradictory personality traits. All except one (Enneagram) recognize the effect of the situational environment of the person. Healthy people are sensitive to their environments. Most personality theories ignores the sociological factors, as well as the psycho-medical situation of the person.