We still do not have a clear account of what depression is at the level that matters for theory or treatment. Not fully, at least. That said, recent integrative work on difficult-to-treat depression, such as Stephen Barton’s, is genuinely promising.
Despite this uncertainty, depression is routinely treated with SSRIs.
One interesting feature of SSRIs is their side-effect profile. Common effects include changes in appetite, sleep disturbance, restlessness or irritability, and fatigue.
This matters because the most widely used measures of depression, such as the PHQ-9, operationalize depression largely through these exact symptoms. In other words, the dominant treatment for depression is often accompanied by changes in the very indicators used to define and measure the disorder. At minimum, this raises serious questions about measurement contamination and construct validity.
This problem is compounded by how depression is defined diagnostically. In DSM-5, major depressive disorder is primarily characterized by depressed mood and anhedonia. These criteria are descriptive, not explanatory, but they leave us with a construct that risks being defined by its own manifestations rather than by clearly specified underlying processes.
Taken together, this may help explain why progress in depression treatment has been slow and why many depression scales struggle to discriminate mechanisms of change. If our measurements are noisy and our construct poorly specified, treatment development will suffer.
This is why we argue, in our paper, that it may be time to return to first principles.
Independent qualitative research on depression could help clarify its core components, their boundaries, and how they relate to one another before we continue refining tools built on shaky foundations.
If you are interested, I have written more about SSRIs here:
And Stephen Barton’s book is an excellent example of the kind of integrative thinking the field needs: https://uk.sagepub.com/en-gb/eur/cbt-for-depression-an-integrated-approach/book255214
