What causes schizophrenia? The short answer may be "nothing" or more precisely "no one thing." In most cases, schizophrenia is an end result of a complex interaction between thousands of genes and multiple environmental risk factors--none of which on their own causes schizophrenia. Daniel Weinberger, in his classic paper on brain development and schizophrenia, entertained the "unlikely" possibility that schizophrenia is "not the result of a discrete event or illness process at all, but rather one end of the developmental spectrum that for genetic and/or other reasons 0.5% of the population will fall into." Over 20 years later, this unlikely scenario is looking more realistic. Schizophrenia is increasingly considered a subtle neurodevelopmental disorder of brain connectivity, of how the functional circuits in our brains are wired. Schizophrenia may in fact be the tail end of a distribution of how the estimated 20 billion neurons and their trillions of synaptic connections in our brains are generated, eleminated, and maintained. Schizophrenia may be the uniquely human price we pay as a species for the complexity of our brain; in the end, more or less by genetic and environmental chance, some of us get wired for psychosis.
This eloquent passage by John H. Gilmore, M.D., from a recent editorial in the American Journal of Psychiatry, struck me as emblematic of the field now. In one sense, his statement seems obvious--we nod knowingly, muttering the mantra that the mind is complicated, etc. But in another sense, it seems to dash our hopes. For what if not only schizophrenia, but most or all mental disorders, are no more easily accounted for than are other complex psychological features, such as intelligence, personality, or the nature of consciousness itself?
Even if we never expected to find THE CAUSE of most mental disorders, the smoking guns, nonetheless we have nourished hopes that one or two of the myriad causes might be found to predominate and to offer chances to nip in the bud "the thousand natural shocks that flesh is heir to." This lust for explanation certainly isn't unique to psychiatrists--patients themselves continually cast about for a simple and compelling narrative, whether it be the abusive parent, the head injury at age 12, or whatever.
Potentially the most embarrassing question psychiatrists are asked is "What caused this?" We still cannot legitimately answer beyond vague references to genes, synapes, and life experience, all of which offers little more clarity than the notorious "chemical imbalance." It may be that we are not only in practice (that is, currently with all our technical limitations), but crucially in theory (that is, forever and by nature of the inquiry) no better able to explain why Johnny is depressed than we can explain why his IQ is 112, or why he prefers baseball to football, or why he likes going to church. The contingency goes all the way down.
Gilmore's editorial also implies that mental disorders may not be contingent afflictions, but may be closely bound up with the very nature of the human animal. It is easy to imagine a world without AIDS (oh right, most of history), and even cancer seems no more integral to human identity than smallpox long must have seemed. But a world in which suicide, madness, and addiction don't happen, period, seems no easier to envision than a world wholly without war or poverty. That isn't to say that it can't or won't happen, but it would involve a radical alteration in human experience.
The murkiness of etiology continues to frustrate the project of neatly carving out mental disorders from the (hopefully) broader region known as normality. All we see wherever we look are continua and shades of gray, and the distinction between treatment and enhancement grows fuzzier. If we had a pill that would increase IQ by ten points, then why would an increase from 50 to 60 be "treatment" (of mental retardation) and an increase from 100 to 110 would be "enhancement" if mental retardation is merely part of the natural (normal?) distribution of intelligence? These distinctions threaten to be made primarily based on pragmatic and political bases (e.g. how many IQ-raising pills can we afford to make, how will be distribute them, etc.).
I've always been struck by the example of Alzheimer's disease, the prevalence of which climbs above 50% in individuals over 85. In this case dementia becomes normative, and statistically an example of "normal" aging. Arguably it is no more a "disorder" than death itself is a disorder. So if we had effective treatments for Alzheimer's "disease," then they could be viewed as an example of "enhancement," as an alleviation of potentially normal aging. What we label "disease" is merely what we would choose not to live with.
In philosophy of mind the "hard problem of consciousness" is the vexed question of how a physical brain produces the experience of subjectivity. As a bad pun I can think of a second "hard" problem of consciousness, that is, the existential reality that consciousness is hard to tolerate at times--it can be raw nerve held up to the universe. But there could be a third kind, a sense in which the brain is evolutionarily hard to produce, the most complex object in the universe that we know of. Mistakes were made. To paraphrase Samuel Johnson's infamously misogynistic (and anti-clerical) comparison of women preachers to dogs walking on their hind legs, one marvels not that it isn't always done well, but that it is done at all.