Not everyone is equally likely to develop psychosis. Since the 1930s we’ve known that large differences in risk exist between different geographical locations and population groups. Young men, for instance, are more likely to develop a psychotic disorder, such as schizophrenia. And ethnic minorities in Western countries are up to five times more likely to develop psychosis, compared with the ethnic majority. Yet we don’t really know why such big differences in risk between people and places exist.
Studying psychosis is difficult. It is a relatively rare group of disorders. Only 1-3.5% of the population will ever be affected, so you need very large studies to investigate differences. We also have to rely on observational studies, which can’t show cause and effect, only associations. It would be unethical – not to mention unworkable – to design a trial where we assign half of a population to live in crowded cities and the other half in the countryside and see if there are differences in psychosis risk between these groups.
Another limitation to studying the geography of psychosis is that there is a distinct Western bias. Our recent meta-analysis, published in The Lancet Public Health, looked at studies reporting the incidence of psychosis (that is, the number of new cases per year). Of the 177 studies that met our criteria for inclusion, only 19 were conducted outside Europe, Australia and North America. This means we don’t understand the burden of disease in many parts of the world, which could have far-reaching implications for what we understand about psychosis.
The association between being born and living in an urban area and psychosis, for instance, has only ever been tested in Western countries. A recent study tested this hypothesis in low and middle-income countries and found no such link. This is only one study, so it’s not enough to make us question everything we think we know, but it does illustrate the fact that the link between psychosis and urban areas might be more context-specific than we have considered so far.
Another factor that makes studying regional variations in psychosis difficult is that there is no consistent way of gathering data. Some studies rely on stats from specialists services while others rely on population registers.
Population registers tend to suggest higher rates of psychosis because they not only count visits to specialised services but across a whole healthcare system. For instance, they include visits to general practitioners.
The relevance of this to geographical variance becomes clear when we look at the so-called “latitude effect”. It is often reported that psychotic disorders are more common at more northern latitudes, but we’ve never discovered a plausible reason why this would be the case.
Coincidentally, a lot of Scandinavian countries also make use of population registers. We don’t know if this explains the latitude effect, but it might contribute to it.
Why it matters
If we had more accurate statistics on the differences in risk, it might tell us something about what causes psychosis. Fortunately, the limitations of the current evidence don’t mean there is nothing we can say about the distribution and risk factors of psychosis. Some of the findings described above, such as the higher risk of psychosis in ethnic minorities in Western countries, are described in so many studies across so many countries that we can be relatively certain it isn’t just a result of differences in study methods.
In a recent study using similar methods to find cases across 17 settings in six countries, we still found an eightfold difference in the incidence of psychosis between the 17 included settings. This was after taking account of the differences in the age, sex and ethnicity profiles of the populations in the different settings. This is important because we would expect a higher rate in an area with, for instance, more young men, because we know they have a higher risk of psychosis. This study suggests that not all variance in risk is due to differences in study methods.
Based on the current evidence, it seems that investigating why psychosis is more common in ethnic minorities in Western countries is a much more pertinent question than why it is more common in Scandinavian countries.