What are the causes of psychiatric disorders? There are several theories, but most mainstream thought in the fields of psychiatry, neurobiology, and related specialties, relates each disease to a combination of genetic and environmental factors. There are also illnesses caused wholly by physical trauma. Many psychiatric diseases might best be considered as syndromes, that is, consistent groups of symptoms that do not always have a single cause.

Diseases from physical trauma

Physical causes include major brain trauma, accidental head injury, and drug use, such as alcohol and narcotics. In addition, certain kinds of toxic substances, such as heavy metals, can induce psychiatric disorders. Lastly, there are illnesses that are not usually classified as mental illnesses that can have psychiatric side effects, such as AIDS-related psychosis. Immediate effects have been conclusively traced to these causes.

There are two important historical examples of psychiatric illnesses traceable to immediate and well-understood physical causes. General paresis of the insane is known to be caused by advanced syphilis and pellagrous insanity is caused by niacin deficiency. Ironically neither of these are any longer thought of as psychiatric disorders, primarily because, as in this case, when a very clear and well-understood physical cause, coupled with a widely-accepted and standard treatment, becomes available for an illness, it generally moves under the heading of another medical speciality, such as neurology.

There are psychiatric problems that are related to heavy exposure to chemical causes, such as amphetamine psychosis, and lead poisoning, which can result in mental retardation. Again, these are often treated by specialists in the field of neurology.

Finally, there are diseases related to immediate head trauma. Bleeding inside the skull can put pressure on the brain, which can result in any number of symptoms, including paralysis, depression, and psychosis.

Environmental and Genetic causes

For disorders such as depression, causation is still somewhat controversial. The most widely-accepted theory is that subtle, possibly hereditary, disturbances in brain chemistry cause depression. The evidence for this view is a statistical correlation between brain chemistry and symptoms. In addition, many studies looking for genetic roots to mental illness, have found strong statistical links showing that certain mental illnesses seem to run in families. Some controversy exists over whether this is due to genetics, or simply from being raised by or with someone who is mentally ill, but evidence from studies examining siblings, especially twins, who were raised separately, seems to indicate strong genetic factors. Further evidence is the apparent success of medical treatment (such as the use of Prozac and other selective serotonin re-uptake inhibitors (SSRIs)), which often results in both changes in brain chemistry and alleviation of symptoms. On the other hand, many studies have found that placebos are equally effective, but it should be noted that placebo effects tend to disappear over time in many patients, and that the placebo effect also exists in treatments for diseases like cancer.

Traditionally various things such as stress, upbringing, and other "environmental" causes have been blamed for psychiatric illness. While such things may possibily influence or induce neurotic illnesses, such as anxiety and minor depression, there is no evidence that there are non-physical causes for major psychiatric illnesses such as schizophrenia, bipolar disorder and psychotic depression. However, there is evidence that, even for psychotic disorders, certain environmental factors may play a role not in causing the disease, but in triggering acute symptoms, in the same way that stress does not cause influenza, but it can exacerbate the symptoms. Psychiatric patients often do not cope well with stress, but that does not mean stress causes psychiatric illness.

For instance, schizophrenia is regarded as an illness with a susceptibility (probably genetic, but in some cases could be due to other things such as a vertically transmitted retrovirus,) and an environmental cause. The majority of environmental causes are likely to be infections (most likely viruses), although toxins remain a remote possibility in some cases. However, the estimates of heritablility of schizophrenia from twin studies varies a great deal, with some notable studies showing rates as low as 11.0 to 13.8% among monozygotic twins, and 1.8 to 4.1% among dizygotic twins ^{1 2}.

People with mental illness may or may not have a characteristic pre-illness personality. Those in which genetic features dominate are more likely to have a pre-illness personality profile (such as the autism spectrum) and those predominantly relate to an environental insult such as infection are more likely to have a normal pre-illness personality. Studies have shown characteristic personality traits in schizophrenia patients 1-2 years before a psychosis but "predisposing" traits are not seen when you look at personality earlier, say 3 - 4 years before first psychosis. This may reflect a slow buildup of environmentally-related damage that finally spills over into full-blown illness.

There are some illnesses, such as post traumatic stress disorder, that clearly relate to some specific stress-inducing event, but even then it is thought that the stressful event simply triggers a disease already latent in the patient's genes, as evidenced by the fact that many people have been raped, been in war, and so on, but only a few of them get stress-related disorders.

Thomas Laughren, who heads the group of scientists at the FDA that evaluates psychotropic medicines, says that "Psychiatric diagnosis is descriptive. We don't really understand psychiatric disorders at a biological level." (source Washington Post) Most mental health care professionals feel similarly, and psychiatric research is a very active field, just like immunology, oncology, and neurobiology.

References

¹Koskenvuo M, Langinvainio H, Kaprio J, Lonnqvist J, Tienari P (1984) Psychiatric hospitalization in twins. Acta Genet Med Gemellol (Roma), 33(2),321-32.
²Hoeffer A, Pollin W. (1970) Schizophrenia in the NAS-NRC panel of 15,909 veteran twin pairs. Archives of General Psychiatry, 1970 Nov; 23(5):469-77.