The intuitive answer is okay as far as it goes. The problem is that it just doesn’t go very far. It begs the question of how a person loses the desire to pursue goals in the first place. The answer involves a surprising theory that takes us closer to understanding how it is that low moods intensify into more serious episodes of depression.

First, we have to detour to contemporary evolutionary psychology, which tells us that moods have a function: Moods help us pursue goals efficiently. High moods help us to more vigrously pursue rewards. Low moods tell us when our progress towards goals is poor. Often, low moods first arise when we’ve hit an obstacle, or when an important goal is threatened. Our usual first reaction to a low mood is to redouble effort towards the blocked goal. If the goal still proves to be unreachable, the low mood will escalate. At some point, something has to give: Usually the person will give up, or scale back on the goal and/or move on to another activity that has a better pay off. Authors such as Randoph Nesse and Eric Klinger have made a powerful case for the utility of low mood. In a world where time, resources, and effort itself are all precious and finite, having an evolved mechanism to hasten disengagement from a failing goal is very important to survival.

These relationships between moods, goal, and effort hold for a variety of species. A bear fishing for salmon without luck in a favorite river bend uses low mood to help it move on to another spot. For better or worse, human self-regulation is more complicated because we can choose either to act or not to act on our mood. I believe that humans are the only species that can decide to ignore low mood and to continue pursuit of an unreachable goal. In a sense, this creates the potential for a stand off between the person and their ancient mood system. To resolve the standoff, the mood system must do something more drastic: It turns down the volume on goal pursuit, not only on the one goal, but on goal pursuit across the board.  Eventually, when the mood system wins the result is flat-on-your-back depression, with fatigue, torpor, a lack of motivation, the whole nine yards.

So this alternative theory turns the standard explanation on its head. Depressed people don’t end up lying in bed because they are undercommitted to goals. They end up lying in bed because they areovercommitted to goals that are failing badly. The idea that depressed people cannot disengage efforts from failure is a relatively new theory. It has not been much tested in research studies. However, the idea is well worth exploring. It fits well clinically with the kinds of situations that often precipitate serious depression — the battered wife who cannot bring herself to leave her troubled marriage, the seriously injured athlete who cannot bring himself to retire, the laid off employee who cannot bring herself to abandon her chosen career despite a lack of positions in her line of work. Seeing these depressions in terms of unreachable goals may be useful clinically, and may help us better understand how ordinary low moods can escalate into incapacitating bouts of depression.”