I think it goes both ways though. We have no real frame of reference for “normal” — most of us only know maybe 20 people well enough to see the inner workings of their mind, and we’re probably related to at least half of them so anything congenital would feel “normal”. It seems pretty common that children of people with undiagnosed mental illness end up feeling weird or crazy until they break free and start to discover the world on their own (see also /r/raisedbynarcissists).

Trauma is a huge factor in mental illness as well, so much so that the definition of a “healthy individual” can only be really anchored to a specific point in time. Some things that rise to the level of clinical (anxiety, depression, BPD, PTSD, DPD) are largely seen as the result of trauma and/or some hereditary predisposition.