Andrew Scull:

Vast resources have been devoted over time to efforts to intervene in, ameliorate, and perhaps cure the mysterious conditions that constitute mental disorder. Yet, two centuries after the psychiatric profession first struggled to be born, the roots of most serious forms of mental disorder remain as enigmatic as ever. The wager that mental pathologies have their roots in biology was firmly ascendant in the late nineteenth century, but that consensus was increasingly challenged in the decades that followed. Then, a little less than a half century ago, the hegemony of psychodynamic psychiatry rapidly disintegrated, and biological reductionism once again became the ruling orthodoxy. But to date, neither neuroscience nor genetics have done much more than offer promissory notes for their claims, as I shall show in later chapters. The value of this currency owes more to faith and plausibility than to much by way of widely accepted science. […] 

But the fundamental point remains: the limitations of the psychiatric enterprise to date rest in part on the depths of our ignorance about the etiology of mental disturbances.