To the best of our knowledge, this is the first mapping review evaluating the geographical distribution of studies examining the association between the most replicated environmental risk factors for psychosis (paternal age, obstetric complications, urbanicity, childhood adversities, migration or ethnic minorities, cannabis use) and incident psychotic disorders.

The first main finding is that the identified studies have been conducted mostly in the Western world with large areas totally unrepresented. Notably, our review shows that out of the 357 cohorts investigating the association between risk environmental factors and psychosis, more than 50% were recruited in just four countries (UK, Sweden, Denmark, US) and most of them in high-income countries. There is a paucity of data on some continents such as Africa, South America and most parts of Asia. Psychological sciences have been criticised for relying on studies conducted in Western, Educated, Industrialised, Rich and Democratic (WEIRD) populations (Burkhard et al., 2021; Rad et al., 2018), which represented the minor part of the world population. Therefore, the risk is of transporting some theories and norms of human behaviour in contexts that differ in terms of ethnicity, cultural and social background, and economic resources compared to those in which they were generated.

The scarcity of evidence on the risk of psychosis in large parts of the world is aggravated by the shortage of genetic studies, the other significant contributor to the causation of psychosis, in non-European populations (Polderman et al., 2016; Tosato and Lasalvia, 2009). Starting with genetic epidemiology research, the most comprehensive meta-analysis (Polderman et al., 2015) demonstrated that the heritability of human traits has been estimated by twin studies from 39 different countries, with a large proportion of studies (34%) conducted in USA, with South America, Africa and Asia heavily underrepresented. Molecular genetic studies are similarly biased, as a disproportionate majority of participants in published genome-wide association studies (GWASs) are of European descent (Peterson et al., 2019) with slow or questionable progress towards diversification the recent years (Fatumo et al., 2022). As a result, potential benefits of genomic research in understanding disease aetiology and improving prediction and clinical care would lead to greater health inequality between different populations and parts of the world. For example, polygenic scores, which have been proved valuable tools for capturing genetic liability to disease, have poor transferability between populations (Martin et al., 2019). Relevant to this review, polygenic prediction of schizophrenia is much higher in European (liability R2 of 7.3%) than in African ancestry cohorts (R2 of 1.7%), demonstrating that any potential clinical implementation is arguably closer in the former compared to the later population (Lewis and Vassos, 2022).

Considering the specific risk factors we included, one of the most extensively studied is obstetric complications (OCs) (Cannon et al., 2002; Davies et al., 2020) due to its central role to the neurodevelopmental model of psychosis (Murray and Lewis, 1988). Studying the role of obstetric complications in different environments is important for two reasons: firstly, the prevalence of OCs differs among world regions; for example, preterm birth is associated with 5–18% of pregnancies, with the highest rates occurring in Africa and North America (Romero et al., 2014). The prevalence of obstetric haemorrhage is higher in Africa (27.7%) compared to North America (13.1%) and Europe (12.7%) (Nathan, 2019), but studies to verify the long-term outcome of both preterm birth and haemorrhage in Africa are lacking. Secondly, the unequal distribution of OCs worldwide is based on the essential maternal healthcare services that are universally used in high-income countries, but overwhelming disparities exist in many countries across Asia and Africa (Yaya and Ghose, 2019). This calls for action to ensure a better perinatal health, especially in low-income countries and raises the question of whether the high prevalence of OCs in countries with reduced childbirth assistance can lead to an increase in the incidence of adverse long-term outcomes, including psychosis.

Most evidence for the association of paternal age with psychosis comes from Scandinavian countries (McGrath et al., 2014; Petersen et al., 2011). In industrialised countries, the age in which one has children is postponed, prioritising educational and career goals. The decision to plan to have children also depends on policies implemented to support parenting and maternal work and the availability of economic benefits or kindergartens. Considering the global increase in education and life expectancy, a trend for delaying parenthood is expected; therefore, it is important to evaluate if late paternal age has a worldwide association with risk of psychosis in children.

Childhood adversities is a complex and heterogeneous phenomenon which has unequivocally been associated with increased risk for mental health problems including psychosis (Baldwin et al., 2023; Varese et al., 2012). According to the World Health Organisation, 36% of children have been psychologically abused, 23% physically abused, 16% neglected, 18% of the girls and 8% of the boys sexually abused worldwide (WHO, 2014). A recent systematic review (Moody et al., 2018) found differences in self-reported lifetime prevalence for different types of childhood maltreatment not only between boys and girls but also among continents. For example, the prevalence of physical abuse is 12% for girls and 27% for boys in Europe, and reportedly higher in Africa (60% and 51% in boys and girls respectively); however, it is difficult to know whether the same criteria have been used to define childhood adversities and to evaluate their psychological impact in different social and cultural environments. The inclusion of low-income countries in psychosis research would allow us to accurately estimate the strength of the association and also identify resilience factors, potentially mitigating the effect of the trauma.

The evidence of association of urbanicity at birth and early development with psychosis comes almost exclusively from Northern Europe (Vassos et al., 2012), where the presence of registers makes possible to link the person’s place of birth to the number of inhabitants in the area. This association has been challenged in low- and middle-income countries where urbanicity at residence was not associated with the prevalence of psychosis (DeVylder et al., 2018). However, incidence studies are missing in large parts of the world. Given that urbanicity is a proxy indicator for exposures that have not yet been identified, examining the geographical distribution of this effect can give us useful insights on the potentially causal underlying risk factors. As nowadays low-middle income countries are undergoing a profound social transformation, with large numbers of people moving into the cities, it is projected that they will soon contain the largest urban agglomerations of population (Kii, 2021). Therefore, there is an urgent need to research if this will come with an increased incidence of psychotic disorders.

There is a robust association between migration or ethnic minority status with psychosis (Cantor-Graae and Selten, 2005; Selten et al., 2020). However, most evidence comes from migration to Europe, usually from similar or lower-income to high-income countries, with very limited data on the opposite direction or on internal migration. The raised rates in second-generation migrants suggest that schizophrenia is unlikely to predispose people to migrate because there is no evidence of an increased incidence in the countries of origin (Bhugra et al., 1996). It can be argued that migration is a proxy risk since other factors as ethnic density in neighbourhoods of the host countries (Boydell et al., 2001) or indicators of social disadvantage (Stilo et al., 2017) have been associated to increased rates of psychosis, as proposed by the socio-developmental pathway to psychosis (Morgan et al., 2010). Thus, if the odds of psychosis in migrants are associated with social vulnerability or the social adjustment/integration in the host country (Tarricone et al., 2021), it is pertinent to test this hypothesis in different regions with different social structures.

Finally, the strong association between cannabis use and psychosis (Marconi et al., 2016; Tosato et al., 2013) is based on more diverse evidence from studies conducted in countries with very different drug-legislation and compounds available (Di Forti et al., 2019). Cannabis remains by far the world’s most-used drug, (UNODC, 2023), and the prevalence of use ranges from 16.6% of the population in the USA to 9.7% in West and Central Africa (UNODC, 2023), while cannabis use is less prevalent in East and Southeast Asia (1.2%) (Kalayasiri and Boonthae, 2023). It should be noted that in the last decade there is a long-term trend of increased THC content (almost by 50%) in seized cannabis herb in Western and Central Europe and the USA (UNODC, 2023), and high-potency cannabis increase the risk of dependence and psychotic disorders (Connor et al., 2021). Similar to other environmental risk factors, expanding research on cannabis and psychosis in areas of the world with different habitual use and THC concentrations would be very valuable.

From our exploratory analysis of the association of the number of papers and recruited cases per country with a variety of potential predictors, the number of researchers per population stands out as the only variable associated with research output when adjusting for all other predictors. The only other predictors nominally associated with research output in 28 countries with at least one publication were the number of Nobel laureates in science and the percentage of GDP invested in research and development. These findings suggest that a country’s focus on research both at baseline (number of researchers employed, R&D spending) and at the higher end (number of Nobel laureates) are among the strongest predictors of published research outputs in the field of environmental risk for incidence of psychosis.

It is remarkable that the total population had no effect on the number of publications. Similarly, the wealth of countries measured as GDP total and per capita, the number of medical practitioners or the number of publications in PubMed during the last decade before covid were not significantly associated with research output in the joint model adjusting for all the predictors. When examining bivariate correlations in the analysis of countries with population over 4 M, it was observed that out of the measures of wealth, GDP per capita rather than total GDP was mostly correlated with the number of relevant publications. Similarly, doctors per population unit rather than total number of doctors and R&D % of GDP rather than total R&D expenditure are mostly correlated with the number of cases. These findings indicate that rather than the total resources of countries, it is the density of researchers or the prioritisation of public spending in research and development mostly associated with research output in this field.

The findings should be interpreted in the context of various strengths and limitations. The scale of this review, examining six well-established environmental factors, which identified 308 studies meeting inclusion criteria allowed us to examine the distribution of published research at a global level and to analyse country characteristics as predictors of the volume of research outputs. We acknowledge that this is a non-exhaustive list but includes some of the most replicated and validated environmental risk factors for psychosis (Vassos et al., 2020). Our mapping review was conducted without linguistic limitations, so the likelihood of missing papers published in non-English-language journals is minimal. It should be noted that the electronic databases we used for our search (PubMed and PsycINFO) are dominated by papers in English language and it is possible that studies from researchers objecting to the globalisation of science may be underrepresented. The heterogeneity between studies in the diagnosis of inclusion did not allow us to draw conclusions regarding schizophrenia specifically, but to study the association between environmental factors and a broader spectrum of psychotic disorders. Finally, a certain heterogeneity in the quality of the extracted data was found because in some papers it was not possible to extract either the timeframe, or the region of recruitment. This lack of information may not have allowed us to correctly identify all the overlapping cohorts, but since this limitation is relevant to a small number of papers, we do not believe that this has affected the bigger picture of our findings.

It is interesting to note that the strongest evidence of association between environmental factors and psychosis comes from Scandinavian countries, such as Sweden and Denmark, which have long-standing comprehensive national and health registers. These countries have a public health system, easily accessible, usually free of charge, where diagnostic and treatment pathways of care are standardised and systematically recorded. In addition, preventative measures and early intervention services potentially influence the incidence and outcome of first episode psychosis. This type of health organisation is not available in a large part of the world, which further highlights the need for a global representation of research on risk for psychosis.