Every single mental disorder predisposed the patient to every other mental disorder — no matter how distinct the symptoms. “We knew that comorbidity was important, but we didn’t expect to find associations for all pairs,” says Plana-Ripoll, who is based at Aarhus University in Denmark.
This might be more accurately framed as “Every single mental disorder [diagnosis] predisposed the patient to [being diagnosed with] every other mental disorder.” With that in mind, I can think of a few causes - with relatively minor disorders (like a mild case of ADHD or a person who has autism but is high functioning), a person who has a single disorder might only have a 50% chance of going to get diagnosed, and the same with a second disorder. If they have one, there’s only a 50% chance of getting diagnosed at all, while if they have both, there’s a 75% chance that they get diagnosed with one, and now that they’re already in the office, they will likely get diagnosed with the second. That goes doubly if the person has a minor disorder and a significant disorder (such as schizophrenia) - they’ll definitely go in for the second, but only might go in for the first, but once they’re getting seen for the second, they’ll get diagnosed with the first.
Huh, this is a really good point. Adam Hunt on twitter also notes:
To explain this link you have to take into account that every mental illness requires a certain level of general dysfunction to meet diagnostic requirements so correlation is inevitable.