Narcissism (or, really, narcisstic personality disorder), like other cluster b personality disorders, can be caused in part by biological factors. For most mental illnesses and disorders, the argument of nature vs nurture is still not fully agreed upon and we lack the ability to prove whether a mental illness is rooted in genetic factors or caused entirely by ones environment or experiences. It's more often than not a combination of both nature and nurture, but environment, support systems or lackthereof, upbringing, and experiences often have a heavier impact on a person's psyche and whether or not they will turn out to develop a disorder. For example, it's said that you're more likely to develop depression, anxiety, adhd, borderline personality disorder, etc. if you have a family history of it, but if you grow up in a very healthy, safe, stable, and supportive environment and don't experience a lot of adversity or trauma throughout your life then it's possible you won't develop anything. Admittedly though, that can also tie into the support you have during or following adverse experiences/trauma as well though, so it gets quite complex to evaluate. I think for any mental illness the causation and outcome is quite nuanced and almost never that simple. Even for a disorder like PTSD, which is caused by experiencing a traumatic event or traumatic events, one could argue that "nature" was still a factor and that your genes/family history could've contributed to the development of such a disorder. In regards to NPD specifically, similar to other cluster b personality disorders, people who develop these cognitive distortions and incurable disorders usually grew up in abusive, traumatic, unstable households. But it does vary and with cluster b personality types specifically it's theorized that there is a decently prominent genetic factor.