From The New Republic:
In a series of papers that Wakefield published beginning in 1992, he developed a theory of mental illness called “harmful dysfunction” (HD). The HD theory holds that disorders are genuine when they meet two criteria: they produce distress or impairment in the afflicted, and they are the result of a failure in a brain mechanism that prevents it from performing its natural function — that is, the function for which it was biologically designed by natural selection. Thus, when a person experiences “normal” sadness, according to the HD model, nothing is broken, except perhaps his heart. Conversely, authentic depression (major depressive disorder) is the product of mechanistic failure. What might such failures be? One hypothesis regarding depression, for example, is that it is caused by a defect in the behavioral activation system. This could account for apathy, dampened interest in both the seeking of pleasure and the person’s capacity to respond to it. What we call panic disorder may have origins in a damaged threat-response mechanism. And some speculate that perhaps schizophrenia is a developmental failure of cognitive processing.
If the specific nature of the dysfunction element of “harmful dysfunction” seems vague, that’s because it is. The simplest scenario would posit an errant gene behind the pathology, but that is not how psychiatric conditions work. Mental illnesses are the product of numerous genes that interact with one another, with the environment, and also with experience. A recent study by the National Institute of Mental Health found that eighty genes could be associated with bipolar disorder, eight of which influence how the brain responds to neurotransmitters such as dopamine. Add to this the miasma of social and personal encounters that impinge upon the genetically vulnerable individual — stress, impoverishment, family instability, drug or alcohol use, and so forth — and the “cause” of mental illness becomes staggeringly complex and elusive.
For more than a decade, the Wakefield HD theory has sparked vigorous debate among philosophers, psychologists, and evolutionary theorists. If disorder is a disturbance in an evolved function “intended” by nature, how can we know what nature intended? Must these defects necessarily impair reproductive fitness to count as a dysfunction? Is it appropriate to rely upon standards of evolutionary fitness that developed under conditions that existed hundreds of thousands of years ago?