Hudson and Pope proposed lumping together this group of mental disorders and symptoms and a few physical disorders (irritable bowel syndrome, migraine) based primarily on putative responsiveness to a group of drugs usually classified as antidepressants, suggesting possible common etiological features.
Hudson JI, Pope HG Jr. Affective spectrum disorder: does antidepressant response identify a family of disorders with a common pathophysiology? Am J Psychiatry. 1990 May;147(5):552-64.