Although the consensus seems to be that narcissism or NPD (clinical narcissism) is a result of abuse or neglect during childhood, there may also be genetic factors involved. An article from The Narcissistic Life cites several studies and concludes that narcissism results from a combination of nature and nurture, describing it this way:
These factors include biological vulnerability, social interactions with early caregivers, and psychological factors that involve temperament. There are studies that suggest that a gene (or genes) for narcissism can be inherited but that a person also needs the “right” environment for narcissism to be manifested.
What this means is that while a child may be born with a predisposition to becoming a narcissist, they won’t unless environmental factors are also fulfilled. If the parents do their job well and give the child a secure emotional foundation, they will not develop NPD even if they are predisposed to it. In this way it works a lot like alcoholism: alcoholics are probably born predisposed to becoming alcoholic, but if they don’t take their first drink until they are well past adolescence or if the culture they are raised in discourages heavy drinking (or drinking at all), they will not develop alcoholism.
Some babies are born more demanding or needy than others. These may be “difficult” children who are easily hurt or upset and have trouble learning self-soothing. Such a temperament doesn’t necessarily indicate the child will become a narcissist, but they are probably more likely to than a calm baby who can soothe themselves, if the parents fail to mirror them properly or don’t attend to their emotional needs.
Most children whose parents were abusive or neglectful do not become narcissists. They may develop some other problem like C-PTSD or BPD or be prone to depression or anxiety instead. These are probably children who have a calmer, less sensitive or less demanding temperament than children who grow up to be narcissists. Personally I think people who develop narcissism were children who were especially sensitive and had no emotional defenses at all so they sent the true self into exile and replaced it with a false one. No other mental disorder causes a person to completely reject their own vulnerability and authenticity.
It’s not always abused children who become narcissists. Some are children who are spoiled by their parents. Spoiling may actually be a form of abuse, because it’s a lie and doesn’t acknowledge the child’s real self. It still fails to mirror them properly. The child is constantly told how perfect they are and showered with gifts and praise for being so “perfect.” As a result, they feel like they must always be perfect which of course is a lie. They feel entitled to whatever they want because of this belief in how perfect they are, and they never learn how to deal with criticism or setbacks when they get out into the real world.
I also think the nature of the abuse and role in the family plays a big part in whether a child develops narcissism and what type of narcissism they develop. Golden children, who are essentially spoiled children, are more likely to become narcissists than scapegoats are. Children who serve as both scapegoats and golden children (common in only children) can also become narcissistic, but I think they’re more likely to become Borderlines. If a scapegoated child does become a narcissist, it’s more likely they’ll become the covert, fragile type of narcissist than the grandiose, overt type.
Some studies have also shown that narcissists’ brains have less grey matter in the left anterior insula region of the brain, thought by researchers to be involved with both the regulation of emotion and the generation of empathy. But the jury is out on whether these brain differences are genetic or if the brains of narcissists fail to develop properly due to being raised in a narcissism-inducing (abusive or spoiling) environment.