Are Writers Narcissistic or Altruistic?

Mary Karr, the award-winning poet and New York Times best-selling memoirist, reportedly opined:

All writers are narcissistic [...] No one can sit in a room by themselves for 12 hours a day thinking about what they're thinking and not be a little more self-focused than the normal person. You're definitely on the far end of the [racist] bell curve. 

And per Poetry School, Sylvia Plath said: 

I think writers are the most narcissistic people. 

But Dictionary.com defines narcissism [ nahr-suh-siz-em ] as an: "inordinate fascination with oneself; excessive self-love; vanity."

And this is how the Mayo Clinic defines narcissistic personality disorder:

Narcissistic personality disorder [...] is a mental condition in which people have an inflated sense of their own importance, a deep need for excessive attention and admiration, troubled relationships, and a lack of empathy for others. 

Thus, I would disagree with Karr's and Plath's assessment that (most) writers are narcissistic. I don't think that (most) writers possess "excessive self-love" and have "a deep need for excessive attention". 

On the contrary, I would opine that (most) writers are altruist, because they "sit in a room by themselves for 12 hours a day", because they desperately want to share their art (with the world).