Although various numbers are bandied about for the degree of risk of violent behavior associated with schizophrenia, they should all be viewed with a degree of skepticism. There are several reasons why it is difficult to put a firm number on this association:
- Schizophrenia is not a definite entity. There is no "gold standard" diagnostic test, it's based on clinical judgment. In research settings, it's often based on something short of adequate clinical judgment, such as a single encounter with a clinician. And the term undoubtedly encompasses more than one disease. The etiology of schizophrenia is unknown, and its associated biology very poorly understood.
- All this means there are big problems with ascertainment bias. Maybe people who commit violent acts are more likely to be diagnosed with schizophrenia, because they come to the attention of potential diagnosticians; or because their behavior itself predisposes to a diagnosis.
- On the other hand, selection bias probably works in the opposite direction. People with major mental disorders who are violent are less likely to agree to participate in studies, more likely to drop out, and harder to follow and recruit.
- Studies based on people who have been convicted of violent crimes may understate the prevalence of schizophrenia because people with mental illnesses are often diverted from the criminal justice system. Also, schizophrenia could affect one's likelihood of being caught and convicted. One would think it would make it higher since the ability to plan and act rationally is impaired.
- Ascertaining violent behavior is just difficult. Reliance on self-reports could obviously bias conclusions either way.
As far as politically motivated, or apparently politically motivated violence is concerned, there are a couple of famous cases that are relevant. John Salvi, who went on a murderous rampage at two abortion clinics near my home, was indeed a devout Catholic who attended anti-abortion protests and associated with anti-abortion activists. He was also without a doubt psychotic. He ultimately took his own life in prison after receiving a life sentence. Ted Kaczynski was diagnosed with schizophrenia by a court appointed psychiatrist, but I find this highly questionable. He clearly has impairments in social skills and empathy, and he's a weird guy, but his thinking is neither delusional nor disordered as far as I can tell. (I don't condone long distance diagnosis and I'm not a psychiatrist, but I just don't think he meets the criteria for schizophrenia so there.) Both of these individuals did indeed subscribe to coherent political ideologies, which were broadly similar to ideas promoted by movements and organizations, although Kaczynski appeared to believe that his thinking was highly original.
But it's a pretty big stretch to say that those ideologies and their champions are to blame for these people's actions. Other anti-abortion terrorists clearly were acting with intent based on the logic of their ideology, but Salvi actually believed that he would be ordained as a priest after committing his crime. And I know of no other deep ecology proponents who mail bombs. Earth First! and similar organizations attack property, but not people. We've heard a lot of nonsense of all descriptions about the tragic events in Tucson, but here's what I think. The guy is completely nuts. Yes, the violent and eliminationist rhetoric coming from the extreme right -- which is the center of the Republican Party -- is atrocious and must stop. But it's a real leap to say it had something to do with this incident.
Update: To clarify, I'm not saying it didn't. I'm just saying we don't know yet. And it is certainly true that all the violent rhetoric that's out there is dangerous in part because it can influence delusional systems such as John Salvi's. I'm just trying to carefully sort these matters out, that's all.