Writing leads to suicide or suicidal people are writers?

From The Atlantic:

When the researchers looked specifically at authors, they found that they are overrepresented among people with schizophrenia, depression, bipolar disorder, anxiety syndrome, and substance abuse problems. Authors were also almost twice as likely to commit suicide as the general population.
 

I think the reason for this is obvious: healthy, happy people have things to do and people to see, but sad people are debilitated by their sadness and isolate themselves with their thoughts...which leads to writing. Not all writers are unhappy, but there seems to be a strong David Foster Wallace-ish strain to the occupation.