The extent of belief in God or gods and adherence to religious ideas varies vastly across individuals and between countries. Why is this? Religiosity seems to have a profound influence on peoples’ behaviour and experiences, including on their physical health, psychological well-being, and quality of life, as reviewed by Koenig (2012). Religiosity is, therefore, of great importance and has strong implications for how people conduct themselves in groups and form their societies. Schizophrenia is one factor that has been implicated in this regard, as it is associated with religious delusions and hyper-religiosity.
That such associations exist makes a great deal of theoretical sense. Schizophrenia is conceived of, in part, as hyper-mentalism. Mentalising involves being observant of external cues of many kinds, and being interested in deducing people’s mental states from these cues. There is a range of severity to schizophrenia-type conditions. Mild symptoms are summarised as ‘schizoid personality.’ This is characterised by anhedonia and apathy. More severe is ‘schizotypal personality,’ where the schizoid symptoms are accompanied by social anxiety, paranoid ideation, unconventional, or paranoid beliefs and, sometimes, psychosis (Hodgekins, 2015, p. 184). Diagnosable schizophrenia is a particularly severe manifestation of these characteristics (Dowson & Grounds, 2006). Badcock (2003) has argued that schizophrenics practice mentalising to a pathological degree. They are obsessed with such cues and are so hyper-sensitive to them that they read too much into them. This means that a frown could be interpreted as murderous intent, leading to schizophrenics becoming paranoid. This extends to how schizophrenics experience the world. They strongly perceive cues of a mind from everyday observation of the world itself. This heightened propensity to perceive associations and patterns also seems to be associated with creativity, in terms of attaining artistic education (MacCabe et al., 2018) and producing academic publications (Dutton et al., 2020).
Empirically, we find that an individual’s placing on the spectrum that has schizophrenia at one extreme is correlated with experiencing religious delusions, as for example in a study of the Xhosa (Connell et al., 2015), and believing in the paranormal (Thalbourne, 1994) and outlandish conspiracy theories (Barron et al., 2018). See Rogers and Paloutzian (2006) for a review. On the basis that religious delusions are associated with religiosity, we would expect schizophrenia to also be associated with hyper-religiosity. One third of schizophrenics are indeed very strongly involved with their local mainstream church, and a further 10% are involved in small sects that tend to be fervently religious, known as New Religious Movements, according to data from Switzerland (Huguelet et al., 2006). These are much higher proportions than in the general population. A systematic literature review looked at finer grained relationships, such as between schizophrenia and types of religiosity, but, similarly, concluded that hyper-religiosity is a robust correlate of schizophrenia (Grover et al., 2014). It is therefore reasonable to assume that a person’s position on a dimension from the lowest level of schizotypal personality to a schizophrenia diagnosis is to some extent associated with that person’s level of religiousness.
This raises the question of whether this association is also reflected at the group level. There are substantial and robust national differences in how religious people in different countries are (e.g. Zuckerman, 2007). This tendency may also include believing in a metaphysical reality, for example. Furthermore, there are differences in religiosity between ethnic groups within multi-ethnic societies, and these persist even when controlling for factors that might influence religiosity, such as socioeconomic status (Kanazawa et al., 2007; Chatters et al., 2009). Controlling for these factors is important because stress, mortality salience, and feelings of social exclusion have been shown to elevate religiosity (Norenzayan & Shariff, 2008). Schizophrenia also seems to be influenced by stress, at least in individuals with a genetic propensity for this condition (e.g. Gomes & Grace, 2017). It might therefore be argued that hardships such as poverty contributes to country level differences in schizophrenia, which would be consistent with the fact that schizophrenia is less prevalent in developed, wealthy countries (World Health Organisation, 2004, p. 35).
There are nevertheless also pronounced differences in religiosity between socioeconomically and culturally similar countries. For example, based on data from 2007, 68% of Finns claim to believe in God, compared to 45% of Swedes (Dutton, 2014, Ch. 12). Similarly, there are national differences in the prevalence of schizophrenia (e.g. Saha et al., 2005) and, in some cases, there are very substantial differences between neighbouring (and socioeconomically and culturally relatively similar) countries, such as between Sweden and Finland, with the Finnish schizophrenia prevalence being double that of Sweden (Suvisaari et al., 1999). Indeed, Finland provides a natural control for nationality, in that 1.5% of Finns have been diagnosed with schizophrenia compared to 0.7% of Finland’s Swedish-speaking minority (Suvisaari et al., 2014). In the USA, it has been found that African-Americans are twice as likely as Whites to suffer from schizophrenia, even when controlling for socioeconomic factors (Bresnahan et al., 2007). The heritability of schizophrenia is extremely high, at the level of about 0.8 (Ekelund et al., 2000; Hiker et al., 2018). One should therefore expect group differences in schizophrenia to follow ethnic lines, which typically coincide with national states (see Salter, 2007). Thus, assuming that a group’s average level of schizotypy is reflected by its schizophrenia prevalence, we would expect the latter to be associated with its average level of religiousness. This is a reasonable assumption, as it has been shown that the higher people score on the schizotypy scale, the greater is their risk of schizophrenia (Lenzenweger, 2018).
Religiosity has decreased rapidly in the industrialised world in recent centuries, which has largely coincided with its economic development. Across countries, there remains a strong negative correlation between religiosity and level of economic development (for a review, see Lynn et al., 2009). We cannot say to what extent the causality goes in one or the other direction, but because of the rapid secular change it would seem likely that any factors associated with economic development, such as higher level of education, better healthcare, stronger rule of law, and higher levels of security might reduce the need for, and deter people from, religious worship. In any case, we must control for the level of economic development, as mentioned above, with a suitable proxy being per capita gross domestic product (GDP). Furthermore, wealth and income are in turn robustly correlated with intelligence, and intelligence is in turn negatively correlated with schizophrenia (for a meta-analysis, see Mesholam-Gately et al., 2009). This association seems to have common genetic influences (Hagenaars et al., 2016), which suggests it would be very strong at the level of nations, which represent the average across huge numbers of individuals who are typically more genetically similar than across country borders (see Salter, 2007). A negative correlation between religiosity and intelligence might therefore be driven entirely by the association between intelligence and schizophrenia, so we have to control for intelligence.
Thus, we test the hypothesis that there is a positive relationship between religiosity and schizophrenia prevalence at the national level, controlling for economic development and cognitive ability.