Schizophrenia and Manic-Depressive Illness: What do We Know about Biological Causes?

By David Phillipi

While the technological advancements of recent decades allow us to map the human genome and look at the brain on the molecular level, the enormous amount of data that has been amassed is virtually useless for psychiatrists trying to diagnose their sick patients because the assumed biological causes of schizophrenia and manic-depressive illness have not been found. No brain abnormalities that are specific to either illness or present in all cases have been identified. Nevertheless, the experts who study and treat schizophrenia and manic-depressive illness (MDI) keep the faith (quite literally) that a breakthrough is just around the corner.

For years, genetic research has appeared to be the most promising of the recently opened avenues, but the excitement seems unwarranted by the findings. The relatively large number of chromosomal regions which may be implicated in susceptibility for bipolar disorder means that hope of finding a specific bipolar gene or even a small number of genes must be given up. Some researchers think the way to go is to narrow the search by looking for genes associated with specific aspects of the disease. Of course, this further refinement is only possible because of the huge variation in symptoms and experiences of those who fall under the MDI/bipolar umbrella, and we are once again reminded of the difficulty of defining what this illness or group of illnesses even is. Furthermore, even the distinction between schizophrenia and MDI seems to collapse in light of the genetic linkage data. In Manic-Depressive Illness: Bipolar Disorders and Recurrent Depression (2nd Edition), Drs. Frederick Goodwin and Kay Redfield Jamison write:

While the search for predisposing genes had traditionally tended to proceed under the assumption that schizophrenia and bipolar disorder are separate disease entities with different underlying etiologies, emerging findings from many fields of psychiatric research do not fit well with this model. Most notably, the pattern of findings emerging from genetic studies shows increasing evidence for an overlap in genetic susceptibility across the traditional classification categories. (49)

Genetic studies in the schizophrenia research community lead to pretty much the same hypothesis as with bipolar: genetic susceptibility is most likely polygenic, meaning dependent on the total number of certain genes which may contribute to vulnerability. It must be noted that genetic vulnerability is a condition, not a cause of schizophrenia and bipolar – something else must be acting on this vulnerability. In one way or another, this fact is usually noted in the literature that deals with genetic data, but it is often obscured by a tone of confidence which suggests the information may be more meaningful and explanatory than it truly is.

Even when a specific gene has been well studied across illnesses, its usefulness in understanding genetic susceptibility may be extremely limited. Some studies in both schizophrenia and MDI have found an increased risk of illness for those who possess the short form of the serotonin transporter promoter gene 5-HTT. The thing is, each of us has two copies of this gene, and over two-thirds of us have one long and one short form, meaning that having the normal variant of the gene is the risk factor! If most of us possess a gene which puts us at risk for an illness which only a small minority of people have, then this particular trait is obviously not much of a causal explanation.

Still today, the most important evidence for the heritability of schizophrenia and bipolar disorder are traditional  genetic-epidemiological studies – “genetic” research only in the sense that we know that relatives share genes.  There is significantly greater lifetime risk of illness for people with a first degree relative who suffers schizophrenia, and studies of bipolar and major depression (i.e manic-depressive illness) have had parallel findings. However, the overwhelming majority of schizophrenics do not have parents or first-degree relatives with schizophrenia, and most of them do not have children themselves, making it difficult to establish the genetic component by looking at family history in a large percentage of cases.

Studies of twins are particularly important for the heritability argument.  Calculations from these studies find a 63% risk of having bipolar disorder if an identical (monozygotic) twin has it. The risk for major depression is significantly lower. In schizophrenia the risk is under 50%. The ideal study design for attempting to separate the contributions of biology and environment involves identical twins, separated at birth, adopted, and raised apart, with at least one of them suffering from mental illness.  As can be imagined, these cases are hard to come by (4 in MDI and 14 in schizophrenia), and the small number of cases makes generalization suspect (though generalizations are often still made). Another method, for which there is significantly more data, is to compare the risks of identical (monozygotic) and fraternal (dizygotic) twins. Because both kinds of twins are assumed to share the same environment, but fraternal twins only share 50% of their genes, the difference in risk between fraternal and identical twins is attributed to genetics. But this method depends on an extremely limited understanding of environment, reducing it to simply having the same parents. It’s likely that identical twins would be treated in very similar ways by their parents and society at large, but fraternal twins, being biologically different (perhaps even in gender) will likely be treated in very different ways. Therefore, it is highly doubtful that twin studies are able to separate the contributions of biology and environment to lifetime risk of mental illness to anywhere near the degree that is suggested. The fact that over one-third of identical twins are not affected by the disease from which their twin suffers reveals again that genetic susceptibility is at most a condition, and not a cause of schizophrenia and MDI.

The prevailing assumption that schizophrenia and MDI have biological causes naturally leads to the expectation of finding them distributed uniformly across cultures and throughout history. In the case of schizophrenia, this belief justifies the adoption of the standard worldwide lifetime risk of 1%, (a nice round number), extrapolated from an embarrassingly small number of studies – one from Germany in 1928, and two from the 1940’s in rural Scandinavian communities. However, there is a serious lack of evidence of the existence of these illnesses before the early modern period, and studies have consistently found significant differences in the rates of mental illness across cultures and between social classes within cultures. Nevertheless, (perhaps because the idea that serious mental illness may affect different populations at different rates does not sit well with us), variations are often explained away with charges of inaccurate reporting and under or over diagnosis. But epidemiological studies sponsored by the World Health Organization carried out over several decades have found that the illness identified as schizophrenia in poorer, “developing” countries tends to be less chronic (fewer psychotic episodes), causes less disability, and has a better prognosis than schizophrenia in more affluent, “developed” societies. Some of the data from Western nations suggests a lifetime risk of schizophrenia greater than 1%, while in poorer societies the number often appears lower. Multiple studies have found the rate of schizophrenia among Afro-Carribeans born in the UK to be higher than the prevalence in the islands from which their families immigrated. Both schizophrenia and MDI have been found to be less prevalent in Asian countries.

Overall, cross-cultural data supports the hypothesis that schizophrenia and MDI are diseases caused by modern culture, and more specifically, that the more anomic a society becomes, (i.e the more identity becomes a matter of individual choice and the less guidance is given by culture), the more mental illness will be found. Research in the U.S has shown a lower age of onset and higher rates of prevalence for manic-depressive illness in those born after 1944 compared to those born before, though this increase has been attributed to the inadequacy of earlier data-collection techniques, which systematically underestimated the true prevalence of affective disorders. Usually, when environment is allowed a causal role in mental illness, poverty and the stress of the urban environment is the safest target to blame, with studies as early as 1939 finding a higher incidence of schizophrenia in lower-class, urban areas. However, when studies began to consider social class of origin rather than merely the status of the patient when the illness was first recognized, the picture changed significantly. The social mobility of schizophrenic patients displays a “downward drift,” suggesting that their greater proportion among the lower class is due to the disability of the disease rather than the stress of this environment. Furthermore, it appears that the upper-class supplies more schizophrenics than could be predicted by the total upper-class share in the population. The majority of studies of manic-depressive illness show significantly lower rates in blacks compared to whites, but this, like so many other findings which make no sense within the biological framework, is dismissed for a variety of reasons as a mistake.

Finally, Goodwin and Jamison tell us that “the majority of studies report an association between manic-depressive illness and one or more measures reflecting upper social class” (169). To explain this finding, they consider the possibility that certain personality traits associated with affective illness may contribute to a rise in social position. (One assumes they mean the occasionally “positive” aspects of mild mania, since it is unclear how crippling depression or delusional mania would aid in social climbing). A second hypothesis, that manic-depressive illness could be related to the particular “stresses of being in or moving into the upper social classes,” is deemed simply “implausible, because it assumes that, compared with lower classes, there is a special kind of stress associated with being in the upper social classes, one capable of precipitating major psychotic episodes.” Furthermore, they accuse such a hypothesis of ignoring genetic factors, though discounting genetic vulnerability as a condition for MDI is quite obviously not implied by this idea.

By now it should be quite clear that the belief that major mental illness is caused biologically has made it virtually impossible to reconsider what the empirical evidence actually tells us. Each time the research that is supposed to support this belief comes up short, it is another occasion for the reaffirmation of faith in a soon-to-come breakthrough. Where the data appears to blatantly contradict their hypothesis, they often simply discount its reliability. While many of the most important experts will freely admit how little we actually understand about mental illness, despite all efforts, it is hard to imagine the direction of these efforts will change much anytime soon. This is not a recipe for scientific progress.

Note: For more, see Liah Greenfeld’s Mind, Modernity, Madness, Chapters 3 and 4.


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