It sure can, but the odds are against it overlooking all of them.
Of course, maybe the genes leading to blindness and schizophrenia together also lead to some other defect that results in miscarriage or some other kind of early death.
Of course, maybe the genes leading to blindness and schizophrenia together also lead to some other defect that results in miscarriage or some other kind of early death.
Congenital blindness is likely diagnosed with near 100% certainty in early childhood. It's easy to spot. The same is very much NOT true of mental health problems. Schisophrenics are diagnosed, almost always, when their condition manifests in such a way to interfere with their life (or someone else's) to the extent that the doctors get called in. There are widely assumed to be LOTS of undiagnosed mental health cases in society, simply because the sufferers live in situations where their disorder can be managed (or suppressed!) in an ad hoc way.
Now, the question becomes: are blind people more likely to be living in a circumstance where their mental health troubles are more easily managed/ignored/suppressed without the involvement of doctors who would otherwise diagnose schizophrenia? That seems not at all unlikely to me.
And frankly: this kind of confounded measurement strikes me as MUCH MORE likely than a heretofore unrecognized link between vision and schizophrenia. Significant results require significant proof, and I don't think this is it.
Could you explain your thinking to me? I'd think that people with one serious condition would me much more likely to have a second serious condition diagnosed, purely because of increased attention from medical professionals. That's certainly been my experience.
Does that apply to blindness at birth, though? I mean, you don't get continuing "treatment" or checkups for blindness, right? (At least, no more than a sighted person would go in for a yearly eye exam.) Obviously there are unique needs a blind person has to function in society, but it's not like something like cancer or a mental illness where you need to follow up and have continuing care and treatment for many years, possibly the rest of your life. For birth blindness, it's basically "yep, kid's blind; need to adapt to that", and that's it from the standpoint of medical care, no?
So let's say someone is blind, and they come in. They will almost certainly be more likely to receive attention than someone who is not. But how their presenting disorganized thoughts, etc. are described is likely to be different given their history of blindness, especially if they had a lifetime history of associated cognitive deficits or issues.
It's also the case that overlapping presence of two pathologies is not necessarily same as them disjointly. So blindness etiological factors + psychosis etiological factors may not equal schizophrenia with blindness, but rather a more severe cognitive and/or physical disorder that receives an entirely different diagnosis. This latter scenario isn't necessarily a measurement issue, but it could be construed that way.
Another thing to consider is that a lot of people with mental illness can tell something is wrong with them because they can compare themselves to others. It's much harder for a blind person to do that because they don't get a lot of the information most people get passively by simply being near other humans.
I have (untreated) asthma, but nobody ever explained to me what having asthma means. I was well into my adulthood before I found out that it is not normal to have difficulty breathing hours and sometimes days after simple aerobic activities. And I didn't even have the issue that I couldn't see that other people didn't have this issue. I just thought that this was normal and they were better trained.
In the kind of first principles sense you are thinking of, sure, proof is proof. But in the real world the burden of proof for an epidemiological headline like this absolutely is much higher, in the practical sense of "should I believe this bullshit or is there a mistake somewhere?"
For example, let's say someone inherits (genetically or environmentally) etiologic factors for psychosis and blindness. It's possible that whatever these factors are, they are severe enough to have widespread enough effects that the person no longer receives a psychosis diagnosis, but instead some other diagnostic label that's more comprehensive in nature.
Schizophrenia is really just a label for a semi-heterogeneous set of pathologies that might be labeled something else. I'm not saying that these problems don't exist, just that how they are perceived and described might vary a bit from scenario to scenario (this is one of the main impetuses for DSM-III and later actually). So someone with one pathology might receive a diagnosis of schizophrenia, someone else, bipolar disorder, and yet someone else developmental disorder NEC/NOS.
It's entirely possible that if there is a negative correlation, it's because one reflects some preventative factors for the other, or something like that, but it's also possible that whatever causes the conjoint presence of the etiologies is catastrophic, leading to death, like you suggest, or the need for some entirely different label.
There are many potential causes of being born blind, not all of them genetic.