The complicating factor is that every disease/disorder/condition (both mental and physical) is a combination of genetics, environment, and the nonlinear interac

The complicating factor is that every disease/disorder/condition (both mental and physical) is a combination of genetics, environment, and the nonlinear interaction of genetics and environment. We can measure the genetic factors, and with somewhat more difficulty, the environmental factors. But that interaction term is extremely difficult to measure. It encompasses things like those mentioned in the article: the fact that "sick" people often don't appear sick unless there is an environmental stressor exacerbating their condition. This kind of conditional effect is very hard to measure scientifically.