Not very, according to Gary Greenberg:
A detailed understanding of the brain, with its hundred billion neurons and trillions of synapses, remains elusive, leaving psychiatry dependent on outward manifestations for its taxonomy of mental illnesses. Indeed, it has been doubling down on appearances since 1980, which is when the American Psychiatric Association created a Diagnostic and Statistical Manual of Mental Disorders (D.S.M.) that intentionally did not strive to go beyond the symptom. In place of biochemistry, the D.S.M. offers expert consensus about which clusters of symptoms constitute particular mental illnesses, and about which mental illnesses are real, or at least real enough to warrant a name and a place in the medical lexicon. But this approach hasn’t really worked to establish the profession’s credibility. In the four revisions of the D.S.M. since 1980, diagnoses have appeared and disappeared, and symptom lists have been tweaked and rejiggered with troubling regularity, generally after debate that seems more suited to the floors of Congress than the halls of science. The inevitable and public chaos—diagnostic epidemics, prescription-drug fads, patients labelled and relabelled—has only deepened psychiatry’s inferiority complex.