The Swedish researchers offer one potential explanation for their results: social drift. Individuals with severe mental illness often have a hard time holding a steady job. Some may turn to self-employment—including in artistic fields. But it’s not clear why this should apply more to writers than to other artists.
Another possible explanation can be drawn from the theory of depressive realism, which essentially claims that depressed people are depressed because they see the world as it is—depressing. They are “sadder but wiser.” Writers have to be careful observers of human nature and society. Painters and composers can take inspiration from suffering; but writers have to: drama comes from misery—comedy, perhaps even more so. Depressive realists may often be drawn to writing for this reason.