With epidemology it's hard to tell the difference, there's no identified mechanism for depression or psychomotor retardation. Perhaps they're in the same categ

With epidemology it's hard to tell the difference, there's no identified mechanism for depression or psychomotor retardation. Perhaps they're in the same category? All psychological disease categories are co-morbid with each other. What about existential crises? Do people who have those also have depression?