I'm not seeing where they are coming up with RNT as a cause, other than a lot of theory. Wouldn't it be a symptom of cognitive decline instead? Dementia patients, particularly those with Alzheimers, tend to become depressed because of confusion and memory loss. Wouldn't it be more likely that these depression symptoms are being caused by deteriotating brain function rather than the other way around?

Of course. It would be bizarre if there weren't a relationship between Lewy Body dementia, Alzheimer's, or vascular dementia (which in old people, means you've gone into heart failure) and repetitive negative thoughts. For one, you know you've got an incurable disease that will inevitably destroy your mind, and you've become one of the rare class of people for which assisted suicide has almost no controversy, it's something you're putting down payments on. For two, you can't finish thoughts.

My father was just diagnosed with Parkinson's a few months ago, and he already has trouble following any conversation, and knows it. If that didn't lead to depression, that's what would be notable. And any insight that he reaches that gives him comfort might be gone an hour later.

It just seems like a silly study.

They don't claim it's a cause. In fact, they explicitly state more research is needed to determine the relationship.

The first thing that jumps out at me is the concept of perseveration (repeated fixed obsessions) that happens in dementia syndromes. It would be interesting to consider whether this is a chicken-or-egg scenario, whether individuals tended to ruminate in earlier life.

No one’s saying anything about it being a cause though … association is not cause

I believe there have been other studies showing people with a history of depression develop dementia at higher rates. There are some that have shown the structural/signal changes that happen after longterm depression as well. These are things that occur years or decades before the dementia.

This is the problem of correlation being reported in media, people read it as "causation found"

When really it's "We've found an interesting association, and we are going to explore it more to see if there's an causation that we can influence"

True, but it still might be a good proxy to estimate how well a client is doing.

Gwern on correlation and causation: https://gwern.net/correlation