Depression raises the risk of depression about 72%. I'm doomed!
I think the lead author's hypothesis that it's vascular changes caused by depression that figure into the later dementia is right on, as well as the severity and length of the depression. It's similar to the link between heart disease and depression, which as been studied as well. Most of the research throws up its hands and says "Chicken, egg!" but personally I'm laying money on the mood disorder being at the cause of it all.