What Causes Narcissism

Narcissism is not a result of being wealthy or being poor, or being very intelligent or having a low "IQ", or from being a certain sex or race or another, or where a person is from, or how they look. It's from weak, injured, or absent boundaries coupled with an attempt to replace healthy self-confidence with "superiority" and entitlement. This boundary weakness and lack of self-confidence can be caused by childhood neglect, abuse, and bullying, and an absence of mentoring and healthy guidance, and also a lack of neutral, accurate feedback during formative years. The superiority behaviors, image displays and self-perception are often picked up from certain adults that the child was exposed to.
People with Narcissism also tend to have less capability of the human brain function called "empathy". Possibly related to the lack of empathy, they also may have a lowered ability to process cause and effect, to do "deductive reasoning", to think critically, and to process abstract connections.

An example of the lowered cognitive ability:
I once had a dog that was a mix between a purebred male Rottweiler and a purebred female black Cocker Spaniel. Both parents were on the premises where the puppies were born, the Rottweiler belonged to the couple who lived there, and the Cocker belonged to their son.
A veterinarian I knew at the time (who had displayed other suspiciously narcissistic behaviors) countered adamantly, with blatant condescension and contempt, that the dog "COULD NOT be a Rottie-Cocker mix, because the two breeds would not and could not mate."
~The most glaring thing about this was that the person was a graduated, practicing Veterinarian. Not a person who doesn't know anything about animals who was just acting rudely, who might not realize that dogs are all the same species (Canis Lupus/ C. l. familiaris). Different breeds are not different species.
This display implied that this person did what they had to do to get through their academic program and obtain their Veterinary degree (VMD or DVM) using memorization, but not very much deductive reasoning, objective observation, or abstract processing. In other words, this "doctor" probably would not be able to diagnose or treat a patient properly unless the patient's ailment or injury FIT and MATCHED something the doctor already knew about and understood.
If a patient came in with something this doctor was unfamiliar with, he or she would likely either treat the patient wrongly, according to some other kind of ailment or injury that was familiar, or turn them away, or say there was nothing really wrong. An N. Human doctor might say that the patient's issues were "all in their head"; a Veterinarian who could not figure out a patient's issue might do far worse.