There are no studies like that, though. It's very very hard to study how a group of people suffering from a disease evolves without treatment, both from an ethi

There are no studies like that, though. It's very very hard to study how a group of people suffering from a disease evolves without treatment, both from an ethical perspective (you can't ethically force people not to seek treatment) and from a practical perspective (it's hard to find a significant cohort of people who recognize they have a disease like depression, do not want to seek treatment for it, but are willing to participate in a clinical study). This is especially true for depression, which manifests most of all as a lack of motivation to do anything in those who suffer from it (often with anxiety at the thought of doing new things). So no one really knows, in a scientific, quantifiable sense, how people who suffer from depression evolve without treatment.