It would seem that if mental illness is a brain illness, mentally ill people would at least do better than someone who has had a mechanically caused brain issue, such as a tumor, an aneurism, a stroke, or a bad blow to the head. All of the latter things can be assessed accurately with medical equipment.
However, many people who have had brain cancer or another obvious brain problem live very productively. For example, Jimmy Carter, former President, recently recovered from brain cancer, and appears to be going strong. John McCain has brain cancer, and I hope that he gets well. He is still functioning as a Senator.
Brain tumors don't usually create the behavioral problems of psychiatric illnesses. Many who have been treated for a head trauma have gone on to be productive, hold jobs, have children, and do all of the normal things.
Perhaps, it is partly an issue of what area of the brain is affected. But I think there is more to it than that.
People generally become diagnosed as "mentally ill" because of some type of behavior issue. In some instances, it is difficult for authorities to distinguish criminality versus a psychiatric problem. Certain things are different, in general. For one thing, a mentally ill person usually can't carry on a normal conversation. That person may be disorganized in her or his behavior. At some point, it will become clear that someone in custody for a nuisance or petty crime is not on a narcotic high as was probably at first assumed, and has a mental illness that caused her or him to act that way.
Some types of obvious brain issues can create behavior problems, such as was first hypothesized about the Vegas shooter. However, much of the time, someone with mental illness has a brain that shows up on brain imaging as normal or close to it.
Yet, medical science believes that mental illness is usually a brain disorder. Psychiatric medication much of the time brings a person back to a semblance of normal. It works for me.
How is it then, that a person with a psychiatric illness who often has an intact brain does worse than someone with a tumor, an aneurism, or a stroke?
In the case of mental illness, these conditions manage to infringe on the operating systems of a person's mind. Most physical brain problems don't touch the operating systems. The person with a physical brain problem will have certain things that don't work any longer. Such as, paralysis, memory problems, speech problems, cognitive impairments. In a schizophrenic all of these things work. Yet, the schizophrenic person, in the absence of treatment, is split off from reality.
Once we are medicated, the psychiatric drugs become part of the disability. Medication may provide relief from some of the worst symptoms of the disorder. However, meds impair the functioning of the brain in general, making it very hard to perform at most jobs.
Mentally ill people, specifically those with schizophrenia, may have impaired functioning in a number of areas in life. Many of us are socially impaired. We may continue to have impaired judgment, including while medicated. We may lack insight into ourselves. And we may be unable to adapt to many environments, and this includes but is not limited to work environments.
I don't have a problem sitting in my home office dreaming up manuscripts to write. However, if my wife wanted me to take her to Lake Tahoe for a weekend, I wouldn't be able to do that. I don't have a problem budgeting what little money I have. However, earning money is problematic, other than the tiny bits that I get for some of my writing. I am not very adaptable.
Socializing is a struggle. I don't dislike people. However, I have a lot of difficulty in social situations.
Here are some environmental factors you should consider: A significant factor that contributes to the impairment of persons with mental illness is that the expectations for us aren't the same as the expectations for a nondisabled person. Secondly, there is outpatient institutionalization, which doesn't prepare a disabled person for a nondisabled work or social environment. Third, we could face rejection of peers we had before we became ill. In this case, we are without the peer support and advice that we might otherwise have. Finally, there is the social stigma in general, in which a mentally ill person can not get hired, and will likely be rejected by prospective mates, unless they too are mentally ill.
To sum it up, there are three significant factors that work against mentally ill people: The brains of most mentally ill people aren't normal; medications to treat symptoms can cause other impairments; and, there are environmental factors.
A person with a head injury or some other mechanical brain impairment must only deal with her or his neurological impairment, and does not usually face the environmental factors, or face brain function being suppressed by psychiatric medications.