I've been inspired to start this thread by a lively discussion that emerged on another thread dealing with a particular person's own challenges caused by long-term depression.

For a while, a few years ago, it seemed like it was becoming a settled issue that the "Chemical Imbalance" theory was close to being accepted as fact. Lately, it seems I'm finding articles that challenge that. Here is one that's a year old:
Does a brain chemical imbalance cause depression? Many find it hard to swallow
I'ld love to know what more members of PC think. I'll try to find more recent articles, if this topic turns out to be of interest here.