What do we know about religion and interreligious peace? A review of the quantitative literature

Abstract

Interreligious relations remain an important dimension of human coexistence and we currently observe an increase in religiously motivated violence and discrimination. Hence, we need to better understand determinants of interreligious peace. Building on a new concept of interreligious peace which includes but exceeds the absence of interreligious physical violence, we provide a systematic review of 83 quantitative empirical studies examining religious determinants of interreligious physical violence, hostile attitudes, threat perceptions, trust, and cooperation. We find that religious ideas foster or hinder interreligious peace depending on their content. Religious identities have negative effects but must be considered in context. Evidence regarding the role of religious practice is mixed and the role of religious actors and institutions remains understudied. Our results show the need for (1) more conceptual clarity, (2) replications in different contexts, (3) research on dimensions of religion beyond identities, and (4) a better integration of different strands of literature.

Contrary to secularization theory, the influence of religion on societies worldwide continues to be strong (Toft et al., 2011; Fox, 2018). In recent decades, there has been an increase in religiously motivated violence—for example, Svensson and Nilsson (2018) observed an increase in violent conflict over religious issues from only 3% in 1975 to 55% in 2015. These developments make the study of determinants of interreligious peace an important object for research.

Current literature on religion, peace, and conflict, however, is mostly concerned with the study of violent conflict and the role of religion in peacebuilding processes (for an overview, see, e.g., Svensson, 2016). Interreligious peace has scarcely been studied, despite a growing body of empirical work investigating interreligious physical violence (Basedau et al., 2016; Kim and Choi, 2017). There are three main implications: (1) it remains underspecified what interreligious peace entails; (2) empirical evidence on potential drivers of interreligious peace is currently scattered across different literatures; and (3) quantitative evidence allowing for generalizations beyond case-study approaches is scarce.

It may be argued that examining how to prevent interreligious violent conflict can help us to foster interreligious peace. However, this assumes that interreligious peace entails only the absence of interreligious physical violence. In line with ongoing debates around conceptualizations of peace (e.g., Söderström et al., 2021), we argue that interreligious peace includes but exceeds the absence of interreligious physical violence. To evaluate what we know about religious determinants of interreligious peace, we compile evidence from the literature on the absence of interreligious physical violence, the absence of hostile attitudes, and threat perceptions between religious groups as well as interreligious cooperation and trust.

Our literature review covers quantitative studies investigating the effect of religious factors on elements of interreligious peace published after 2000 as peer-reviewed journal articles. Generally, we find that religious ideas foster or hinder interreligious peace depending on their content. Religious identities mostly have negative effects but must be considered in context, and religious practices may signal trustworthiness, but participation in such practices does not impact a person's attitudes relevant to interreligious peace. The role of religious actors or institutions remains understudied, but they may have positive effects on interreligious peace.

In the following, we, first propose a conceptualization of interreligious peace, enabling empirical investigations of interreligious peace. Second, we compile and evaluate evidence on interreligious peace currently scattered across disciplines. Third, we highlight research gaps and outline ways forward for future research.

Conceptual and theoretical considerations

How do we understand interreligious peace?¹

We consider interreligious peace to be relational (Söderström et al., 2021), not bound by location and existing in the relationships between religious institutions, groups, and individuals. Within a given space, interreligious peace may exist between some religious groups but not others. For example, a religious group may coexist in a peaceful relationship with one religious group in a country, while it may be simultaneously engaged in an armed conflict with another within the same country. Interreligious peace extends to relationships between major religious groups like Christians and Muslims, but also between denominations like Catholics and Protestants as well as religious and explicitly non-religious groups like atheists or agnostics.

In congruence with debates around general peace, we argue that interreligious peace includes but exceeds the absence of physical violence as this is more instrumental to describe variations in interreligious relations. As others, we contend that peace as the absence of physical violence is blind to nuances of empirical realities (e.g., Goertz et al., 2016; Ossai, 2020). The absence of physical violence is merely the smallest common denominator of what peace entails and many ideas coexist regarding what peace entails beyond the absence of violence (Davenport et al., 2018). Our concept builds on previous literature, particularly relational peace concepts (Kasten, 2017; Söderström et al., 2021) and is closely related to discussions around (religious) pluralisms (Joustra, 2020). In contrast to pluralism debates focusing on processes that enable the coexistence in unity within diversity, we focus on defining what the end-state entails. Further, we extend existing concepts of relational peace by reducing the number of constitutive factors and by referring particularly to relations between religions. Overall, we aim to formulate a definition that is concise, recognizes that peace is more than the absence of physical violence, does not prescribe a specific institutional form, and can be operationalized for qualitative and quantitative empirical investigations.

We differentiate between four elements of interreligious peace depicted in Table 1. Negative interreligious peace describes aspects that need to be absent, whereas positive interreligious peace refers to elements that are required in peaceful interreligious relations. Additionally, we maintain that behavioral and attitudinal dimensions inform relationships in line with other concepts of relational peace. Based on these reflections, we define interreligious peace as the absence of physical violence, hostile attitudes, and mutually perceived threat, as well as the presence of trust and cooperation in interreligious relations. Hereby, physical violence refers to physical harm that may take various forms and is perpetrated by adherents of one religious group against adherents of another. Hostile attitudes are hateful, aggressive, or harm-wishing attitudes that can be considered the mental equivalent of physical violence while mutually perceived threats imply that damage or hurt is expected to emanate from the other. On the side of positive interreligious peace, trust entails that actors hold optimistic expectations about the behavior of religious others toward them and their religious group. Cooperation refers to actions undertaken by individuals of different religions to work together toward a common goal.

Table 1. Dimensions of positive and negative interreligious peace

How do we understand religion?

Like peace, we conceive of religion as a multidimensional concept. We understand religion as a set of interrelated ideas in which supernatural factors explain the origin and functioning of the world and provide the meaning of life (Basedau et al., 2018). Four real-world dimensions of religion seem particularly relevant for the analysis of its role for interreligious peace: religious ideas, religious identity groups, religious practices as well as religious actors and institutions (see Basedau et al., 2018). Religious ideas comprise, for example, beliefs, values, formal and informal norms, or ad hoc interpretations in relation to desired social conduct. Once people share a belief and differ from others in that regard, they form groups and are hence subject to dynamics between (religious) identity groups. Religious practices refer to specifically religious behavior like worshipping, praying, following specific dietary rules, or making pilgrimages. Religious actors and institutions include individuals like clerics, organizations of individual religious groups or coalitions of several religious groups as well as state or international institutions that regulate relations between religious groups, and between the state and religious groups or faith in general.

Literature review

Our literature review covers quantitative studies investigating the effect of religious factors on elements of interreligious peace published after 2000 as peer-reviewed journal articles.² While these criteria restrict our understanding of the literature, they are necessary to delimit our work. It is unlikely that substantial research is excluded by focusing on research published after 2000 since only nine of the 83 reviewed studies were published prior to 2010, none prior to 2005. Our multidimensional understandings of religion and interreligious peace provided us with a structure for our search for literature and for the presentation of our review below. We searched for and grouped studies according to outcome variables reflecting dimensions of interreligious peace. We initially identified literature through keyword searches in large scientific databases. In a second step, we surveyed literature cited by and literature citing a relevant study. We included empirical journal articles that use statistical methods for analyses, that is, quantitative studies, that examine relations between an independent variable measuring a dimension of religion, that is, religious ideas, identities, practices, actors, and institutions, and a dependent variable measuring an element of interreligious peace, that is, interreligious physical violence, hostile attitudes, threat perceptions, cooperation, and trust.

These parameters imply that we exclude literature on the role of religious actors in peacebuilding processes like mediation processes (e.g., Johnstone and Svensson, 2013) as well as the role of religion for non-violent protest movements (Svensson, 2016). Peacebuilding and non-violent protests are conceptually distinct from interreligious peace as our key dependent variable of interest. Moreover, excellent reviews on terrorism research and radicalization processes exist (e.g., Schuurman, 2019; Vergani et al., 2020) and we only include terrorism studies if they clearly focus on interreligious violence, thereby excluding (1) literature that does not distinguish between the religious identity of perpetrator and target and (2) studies that focus on terrorism more generally (fewer than one-third of all identifiable terrorist attacks have a religious connotation, see Saiya and Scime, 2015). We do not cover literature on religion and prosociality per se (see Hoffmann, 2013, for an overview), but only review studies investigating aspects of prosociality relevant for our interreligious peace definition (i.e., trust and cooperation).

Methodological challenges of studying interreligious peace

In total, we review 83 studies (see Appendix A: Reviewed Literature). Based on the profile of the reviewed literature presented in Table 2, we identify several methodological challenges. First, the literature on (aspects of) interreligious peace is currently scattered across disciplines. Being mostly published in psychology (37), political science (17), and economics journals (13), discourses around the same social phenomenon usually take place in bubbles that do not cross-reference papers beyond their own discipline.

Table 2. Profile—literature on interreligious peace^a

^a Some articles discuss studies conducted in several countries, on multiple samples or employ different empirical strategies and are considered in two sub-sections of the literature review.

Second, availability of global and large N data remains limited. Only 10 reviewed studies focus on the global level. While there are quite a number of case studies, for example, on Northern Ireland (e.g., Balcells et al., 2016; Adida et al., 2022) or on India (e.g., Field et al., 2008), studies on the global level remain the exception. Comparative designs and replications are important to better understand how context factors matter. Most studies were conducted in the United States (24), primarily on distrust against atheists. A large proportion of studies have been conducted on the Asian continent while particularly research on (aspects of) interreligious peace in South America is very rare.

Third, 66 of the reviewed studies focus their analysis on the individual level. While this is not a problem per se, taking also into consideration that most studies investigate their research question with a student sample, generalizations beyond the case studied become challenging. Many studies do not report how they sample their research participants or rely on a convenience sample of online respondents, which makes generalizations even more difficult.

Fourth, the empirical strategies employed differ substantially across studies and limit comparability: in the experimental literature, behavioral experiments such as the trust game or prisoner's dilemma experiments aim to measure behavioral differences (e.g., Chakravarty et al., 2016). In contrast, studies employing survey designs often ask about attitudes (e.g., Asadullah, 2017) while cross-country analyses focus, for example, on violent conflict events (e.g., Basedau et al., 2016). Thus, it is likely that studies tap into different aspects of a phenomenon. While converging findings across indicators may highlight their robustness, differences may result from different empirical approaches.

Dimensions of interreligious peace

In the following, we review the literature on each dimension of interreligious peace separately, that is, interreligious physical violence, hostile attitudes, and threat perceptions as well as interreligious trust and cooperation.

Interreligious physical violence

Theories on the influence of religion on conflict argue that religious conflicts are special and seem more intractable vis-à-vis other types of conflicts due to the belief in supernatural rewards and punishments (potentially also in an afterlife) and the indivisibility of religious claims (e.g., Hassner, 2003; Ellingsen, 2005). Religion can also be a driver of other types of conflict, such as ethnic conflict, for example, when threat perceptions are prevalent (Fox, 2000). However, it is not clear to what extent it is the major cause of such conflict as other differences, such as linguistic differences have been shown to make intrastate conflict more likely as compared to religious differences (Bormann et al., 2017). In a process of religious outbidding, secular conflicts are sometimes reframed to religious ones (Toft, 2013) and conflicts between believers of the same faith emerge to (re)define religious and political authority (Zellman and Brown, 2022). Similarly, religiosity may matter for conflict initiation (Alexander, 2017). While empirical studies find that religious conflicts last longer as compared to non-religious ones (e.g., Nilsson and Svensson, 2021; Deitch, 2022), results on the impact of religion on conflict termination, re-occurrence, and the level of violence are less clear (e.g., Toft, 2021; Deitch, 2022). For this review, we explicitly focus on the link between religion and violent conflict between different religious communities, thereby excluding excellent papers which do not consider interreligious conflict as the dependent variable.

We identify 14 papers seeking to explore the relations between religion and interreligious physical violence. They are published in political science (12) and economics journals (2) and mainly based on secondary data. While some studies focus on the global level (e.g., Basedau et al., 2017; Kim and Choi, 2017), others investigate case studies such as India (Field et al., 2008) or Northern Ireland (Adida et al., 2022). Table A1 in the Appendix presents a profile of the papers covered.

What do we know about religion and interreligious physical violence? Existing theories and their empirical support are visualized in Table 3. Only one paper explicitly focuses on the impact of religious ideas on interreligious physical violence: Basedau et al. (2016) theorize that religious elites promote specific ideas and thereby make them theologically justified, beneficial, or even imperative (A1). Considering 130 countries over the period 1990–2010, they find indeed evidence that calls for violence by religious elites are associated with increased interreligious conflict onset. However, the direction of causality is unclear as calls for violence could also result from experiencing previous violence (Isaacs, 2016). Other, or more specific, religious ideas have not been tested.

Table 3. Interreligious physical violence: theory and empirical support

Studies on the impact of religious identity on interreligious conflict hypothesize that religious demographics affect the opportunity for collective action or intergroup competition (A2). For example, Balcells et al. (2016) expect violence to be more likely when communities are of similar size (polarization) due to increased competition and threat perception. Basedau et al. (2011, 2016) expect religious fractionalization to reduce and dominance of one religious group as well as religious polarization to increase collective action and thus, interreligious physical violence. Other authors suggest more complex, for example, curvilinear, relationships between religious demographics and violence (Klašnja and Novta, 2016; Adida et al., 2022).

Many empirical studies find that religious demographics matter for interreligious violence (see A2, Table 3). However, the exact form of this association remains unclear as, for example, Adida et al. (2022) find the relationship between diversity and violence to be curvilinear, with the steepest increase when diversity increases from very low-to-medium levels, supporting theories of group threat through exposure. In their example from Northern Ireland, Field et al. (2008) show that historical conditions (in this case, tenancy rights) can shape segregation and religious demographics. Thus, other context factors (such as political, economic, and historical conditions) seem crucial in shaping how religious identities matter for interreligious violence.

Basedau et al. (2011) find that particularly in environments of pre-existing tensions between or discrimination against religious communities, religious polarization goes along with interreligious physical violence. Theoretically, identities running parallel (“overlapping identities”), for example, religious communities identifying with the same ethnicity (or another relevant identity group) can increase violence toward (non-overlapping) outgroups as they facilitate the opportunity for collective action (Basedau et al., 2011; Basedau et al., 2016). Empirical evidence supports this hypothesis, emphasizing the importance of religious identities when they run in parallel with other identity groups.

We could not identify literature examining the impact of religious practice on interreligious violence. When it comes to religious actors and institutions it is assumed that a politicization of religion makes violent conflict more likely due to power imbalances and/or a culture of intolerance (A4). At the same time, felt discrimination and marginalization are expected to increase the motivation for violent collective action (A5). Empirical results support these theories (but come from case studies only): for example, in their study on Nigeria, Bunte and Vinson (2016) find evidence that power-sharing of different religious groups reduces the number of interreligious violent events. In their case study on India, Dhattiwala and Biggs (2012) find a strong association between electoral competition and interreligious violence between Hindus and Muslims. Grievances over discrimination are found to be (weakly) associated with interreligious conflict onset in one study (Basedau et al., 2016), but not in another (Basedau et al., 2017). In his model on competition over religious adherents where religious organizations are treated as rational firms, Isaacs (2017) expects religion to be more salient in conflicts when groups' religious organizations are fragmented (A6). He finds support for his hypothesis (see also Zellman and Brown, 2022).

Finally, the level of religious restrictions may matter (A6), see also Saiya (2018) as well as Saiya and Manchanda (2020): in their study on autocracies, Kim and Choi (2017) hypothesize that the probability of interreligious violence is particularly high when rulers enforce moderate restrictions on religion. They find evidence for their hypothesis as there is no possibility for violence when restrictions are tight and no reason when they are loose, supporting the idea of a non-linear relationship.

Taken together, these findings suggest that (1) religious ideas may affect interreligious violence both positively and negatively but empirical evidence is limited; (2) religious demographics impact interreligious violence, especially in interaction with other unfavorable conditions; (3) there is a lack of evidence on the association between religious practices and interreligious violence; and (4) the existence of religious institutions creating equality and power-sharing between religious groups; competition between religious groups; as well as religious restrictions matter for interreligious physical violence.

Hostile attitudes and threat perceptions between religious groups

Next, we review the literature on hostile attitudes and threat perceptions between religious groups. We include literature that aligns with our understanding of hostile attitudes as unfavorable, mostly emotionally charged attitudes toward another party such as aggression, the wish or intention to harm someone, excluding broader literature on discrimination and prejudice. We identify 22 papers investigating hostile attitudes (13) and threat perceptions (12) as dependent variables. The majority of them are published in psychology journals (12) (see Table A2 in the Appendix). All except for one study focus on the individual level and most papers investigate one specific country context. The reviewed studies mostly use survey designs (19), while three studies employ experimental approaches. Thus, statements about causality are—in many cases—not possible.

Table 4 summarizes some of the theories tested and their empirical support. Fundamentalist religious ideas are assumed to increase hostile attitudes toward other religious groups as they usually emphasize an exclusive truth-claim of religion (B1). This idea is supported across studies (see B1, Table 4) hinting at a certain robustness of this finding. We could not identify literature focusing on more inclusive religious ideas.

Table 4. Hostile attitudes and threat perceptions: theory and empirical support

Two studies test if religious identity impacts hostile attitudes: they find an association between the two if violent-prone ideas are prevalent and/or believers feel threatened by others, see B2 in Table 4. Furthermore, ingroup identification seems to increase hostile attitudes and threat perceptions (B3) due to intergroup biases as shown by several studies (see B3, Table 4). Drawing on case studies from France (Badea et al., 2020) and the United States (Gerteis et al., 2020), it becomes evident that threat perceptions are particularly high if people believe that their national/political identity is tied to religion (B4). Studies by Kanas et al. (2015, 2017) as well as Obaidi et al. (2018, 2022) demonstrate how threat perception can lead to hostile attitudes, for example, through fears of replacement of the own religion by another one (B5).

Contact between religious groups is thought to decrease hostile attitudes and threat perceptions under some circumstances (B6), for example, when it is institutionalized and structured. Empirical evidence shows that contact can indeed reduce hostile attitudes and threat perceptions (see B6 in Table 4). However, reverse causation may also be at play as higher perceptions of threat seem to reduce the quantity and quality of contact (Kanas et al., 2016).

Theoretically, religious practice can impact hostile attitudes in differing ways (B7a and B7b)³: attending service can decrease hostile attitudes if it has beneficial effects on prosocial values, for example, when service contents transmit prosocial ideas but also when common activities such as volunteering play a role. Yet, it could also increase hostile attitudes as the ingroup identity is emphasized through service contents and common rituals, which could enhance outgroup derogation. Furthermore, service attendance could be interpreted as commitment to conservative and authoritative values. Empirical evidence is mixed: while two studies find a negative association between service attendance and hostile attitudes and threat perceptions, two find a positive association and three studies do not find any significant association (see B7a and B7b, Table 4). These differing results could be due to differences in operationalizing service attendance (which is often subsumed under “religiosity”) as well as different study contexts (e.g., Bangladesh, Kenya, Malaysia, Switzerland, the United States, or a subset of different countries).

Only one identified study empirically tests the connection between religious actors and institutions and hostile attitudes. Neuberg et al. (2014) find support for their theory that when religion is an important part of public life, hostile attitudes increase if imbalances between groups regarding power and resources exist (B8).

To sum up, empirical evidence points at (1) the importance of religious ideas for hostile attitudes toward religious others. (2) Religious identity impacts hostile attitudes and threat perceptions, particularly in interaction with other factors, for example, the prevalence of fundamentalist religious ideas or power imbalances. (3) Religious practice is not robustly connected with attitudes toward others. (4) Religious actors and institutions seem to be important for hostile attitudes, but empirical evidence is scarce.

Interreligious trust

Turning to interreligious trust, the 42 articles reviewed investigate trust in the relation between members of different religious groups but also between religious and non-religious individuals. With few exceptions, studies on trust between religious and non-religious are conducted in the United States whereas studies on trust between religious groups are mostly set in Asian contexts with many studies focusing on China, Bangladesh, and India. All studies considering relations between denominations were conducted in Northern Ireland. Most studies are based on psychological or economics research, or the intersection between the two. Measures of trust depend on the empirical strategies employed which include behavioral experiments, survey experiments, and surveys (see Table A3 in the Appendix).

Table 5 provides an overview of theories on religious determinants of interreligious trust and their empirical support. Regarding religious ideas, a prominent theory is that belief in a (monitoring) God increases distrust in atheists (C1). The idea is that others believe religious individuals to be acting under the impression of being monitored by a rewarding and punishing God, which is thought to make them more prosocially (Gervais et al., 2011; see Gervais, 2013). Particularly individuals who believe in a (monitoring) God should, hence, distrust atheists, because atheists do not believe in the existence of God and consequentially are expected to lack prosocial behavior. Empirical evidence supports this theory (see C1, Table 5). Alternative mechanisms suggest that effective secular institutions may replace the effect of a moralizing God (Norenzayan and Gervais, 2015) and that religious identities are associated with specific reproductive strategies, which signal trustworthiness (Moon et al., 2018; Lambert et al., 2023).

Table 5. Interreligious trust: theory and empirical support

Most studies investigate the impact of religious identities on interreligious trust. The intergroup bias hypothesis (C2a) builds on the idea that a common social identity like a religious identity suggests familiarity, similarity, connectedness, or closeness which result in greater trust in the ingroup and lower trust in the outgroup. Empirical evidence is mixed (C2a, Table 5): some studies report significant intergroup bias, while others do not. Four reasons could explain these diverging findings. First, design differences must be considered. To illustrate, Chuah et al. (2016) report that religious people believe that other religious people are more trustworthy than non-religious individuals, but they do not act on this belief in their trust game (see also Johansson-Stenman et al., 2009). Thus, religious ingroup favoritism seems to show less in behavior than in attitudes. It may further matter how religious identities are conveyed as this is not uniform across studies (Timming and Perrett, 2016; Galen et al., 2022; Totton et al., 2023). Second, only established identity groups show ingroup favoritism. Although Christians rather consistently show ingroup favoritism, groups subsumed under “non-Christian,” “non-religious,” or “people believing in karma” do not trust those falling under the same category more (e.g., Xu et al., 2018; Grove et al., 2020; Ong et al., 2022). Moreover, studies must specifically match religious identities of targets to respondents as, for example, only atheists may favor atheists but not all respondents that are categorized as non-religious. Indeed, diversity within the non-religious should be considered in future studies (e.g., Thunström, 2020; Van Tongeren et al., 2023). Third, the intersection of religious with other identities must be considered. Intersecting identities like nationality (Hall et al., 2015) or student identities (Delavande and Zafar, 2015; Galen et al., 2022) could bridge religious differences but also be more salient in determining (dis-)trust rendering common religious identities irrelevant (Howard, 2022).

Fourth, context matters beyond identity salience and intersectionality. Particularly, demographic constellations and minority–majority relations impact interreligious trust (Gervais, 2011; Gupta et al., 2018). Further, interreligious trust in the relation between Protestants and Catholics in Northern Ireland was found to be improved by experiences of positive interreligious interactions (C4, Table 5). Higher levels of desegregation and larger minority groups increase the frequency of (positive) interreligious interactions (Wagner et al., 2006; Kenworthy et al., 2016; Laurence et al., 2018). Therefore, demographic constellations, resulting majority–minority relations and opportunities for positive interreligious contact should be considered when studying interreligious trust.

Further, religious identities could be associated with interreligious trust if higher religiosity implies stronger ingroup identification and enhances biases in interreligious trust (C2b). Studies that find significant effects report ingroup trust rather than outgroup distrust to be amplified. Yet, mixed results (C2b, Table 5) remain difficult to evaluate as religiosity is operationalized in various ways—not always clearly measuring strength of ingroup identification—and theorizing remains limited (for a discussion on religiosity and generalized trust, see Badaoui, 2023). Moreover, cross-national studies suggest differences depending on region and religion (Kollar and Fleischmann, 2022; Badaoui, 2023). Only a measure of religiosity that taps into exclusivist ideas rather than strength of identification tends to be negatively related to outgroup trust (LaBouff and Ledoux, 2016; Kollar and Fleischmann, 2022; Masood et al., 2022; Badaoui, 2023).

Finally, religious identities may signal trustworthiness by signaling a belief in a monitoring God (cf. C1, Table 5) to religious and non-religious individuals alike as well as across religious divides (C3, Table 5). Mixed evidence suggests that effects of generalized trust into religious people (C3) superimpose with intergroup bias effects (C2a). For example, Chuah et al. (2016) find that religious and non-religious alike believe that the more religious are more trustworthy while—at the same time—ingroups are favored. Similarly, Edgell et al. (2006) report that although atheists show distrust against atheists, they do so less than religious people. Further investigating the interaction between the two effects could advance the study of the effect of religious identities on interreligious trust.

A fruitful way forward may be costly signaling theory (Ruffle and Sosis, 2007), which suggests that participation in religious practices signals trustworthiness as it shows commitment to the group and its traditions as well as acknowledgment of accountability (C4). There is evidence that individuals participating in religious rites are more trusted than those who do not (Purzycki and Arakchaa, 2013), even across religious divides (Hall et al., 2015, experiment 4). Yet, replication of these results in different contexts is warranted. Studying effects of participation on religious practices seems less relevant as frequencies of praying did not affect interreligious trust in previous studies (e.g., Johansson-Stenman et al., 2009; Shaver et al., 2018).

The role of religious actors and institutions in interreligious trust is not considered in the reviewed literature. Yet, a study by Kenworthy et al. (2016) examined perceptions of equality in power of Catholics and Protestants in Northern Ireland and found a positive relation with outgroup trust. Future studies should investigate if religious actors and institutions could establish perceptions of equality between religious groups and whether these findings hold in other contexts.

Overall, the reviewed literature indicates that (1) belief in a (monitoring) God has negative effects for trust between religious and non-religious. (2) Established religious groups favor their ingroup but intersecting identities and context factors must be considered. (3) Participation in religious practices seems to signal trustworthiness—potentially across religious divides. (4) The role of religious actors and institutions in interreligious trust warrants research.

Interreligious cooperation

We reviewed eight studies on religious determinants of interreligious cooperation. Most of these studies are based on economics or psychological research, study cooperation at the individual level with students, and employ economic games or survey experiments (see Table A4 in the Appendix). One exception is a study on interreligious networks in developing countries at the country level (Vüllers et al., 2015). Conducting research outside artificial laboratory settings, beyond student populations, and in more geographical locations could generate greater insights on the generalizability and real-life applicability of existing research.

Table 6 shows an overview of theories on religious determinants of interreligious cooperation and their empirical support. Concerning religious ideas, evidence is mixed whether fundamentalist convictions decrease interreligious cooperation (D1; Chuah et al., 2014; but Xia et al., 2021). Theory remains rather unspecific expecting fundamentalist convictions to affect adherent's social interactions generally and, hence, also interreligious cooperation specifically (Chuah et al., 2014). Developing hypotheses detailing why fundamentalist convictions reduce interreligious cooperation may allow for more specific theory testing. The impact of religious ideas may depend on their content (D2). Indeed, Preston and Ritter (2013, experiment 3) report evidence for their hypothesis that concepts of God highlight outgroup prosociality norms and, hence, increase interreligious cooperation. They also find that being primed with the concept of religion decreases interreligious cooperation potentially because it highlights ingroup favoring norms. This suggests that the specific content and potential imperatives of religious ideas determine their effect on interreligious cooperation.

Table 6. Interreligious cooperation: theory and empirical support

Having different religious identities does not per se have a negative impact on cooperation. While there is evidence that cooperation with religious others is lower than religious ingroup cooperation (D3a, Table 6), interreligious cooperation does not seem to differ from cooperation with unknown others (D3b, Table 6). Hence, religious identities lead to ingroup favoritism (ingroup cooperation > cooperation with others), but not outgroup derogation (outgroup cooperation < cooperation with unknown others).⁴ Moreover, Keuschnigg and Schikora (2014) find that participants do not contribute less to mixed than homogenous groups in a public goods game. Chakravarty et al. (2016) only find evidence of ingroup favoritism in contexts with a certain level of religious fragmentation and argue that ingroup favoritism only manifests if religious ingroups are salient. Similar to the discussion on biases in interreligious trust, studies on biases in interreligious cooperation must consider context factors, specifically demography, and the salience of identities studied.

Religiosity is sometimes understood as an indicator of an individual's strength of identification with a religious group. Strong identifiers are expected to have a greater motivation to uphold a positive group identity through outgroup derogation which would decrease interreligious cooperation (D4; Chuah et al., 2014). There is no evidence supporting this hypothesis (D4, Table 6). However, religiosity measures used in these studies vary and may not always be indicators of strength of identification with or commitment to the ingroup.

Another hypothesis suggests that religious individuals may be intuitively cooperative regardless of the cooperation partner's religious identity (D5; Isler et al., 2021). In contrast to this claim, Isler et al. (2021) find that Christian participants cooperate more with fellow Christians than atheists (ingroup favoritism). They further report intuition to decrease interreligious cooperation while deliberation increased cooperation also across the religious–non-religious divide.

The effects of religious actors and organizations as well as of religious practices on interreligious cooperation are not studied except for an initial study investigating the impact of religious leadership (D6; Keuschnigg and Schikora, 2014). Opposite to expectations, religious leadership did not enhance interreligious cooperation. Keuschnigg and Schikora explain these findings with “bad leadership” (leaders contributed less than average contribution to the no-leadership control group). Through this study, we do not learn about potential effects of “good leadership.” Moreover, in this experiment religious leadership was operationalized as one participant contributing prior to the others which may not reflect the role of religious leaders in real life.

All in all, the scarce literature on the effect of religious factors on interreligious cooperation suggests that (1) the role of religious ideas for cooperation depends on their content. (2) Interreligious cooperation is smaller than ingroup cooperation but there is no evidence of outgroup derogation, that is, that interreligious cooperation is less prevalent than cooperation with unknown others. (3) Rather than religious individuals being intuitively cooperative, deliberative thinking potentially enhances interreligious cooperation. (4) The role of religious practices as well as of religious actors and institutions in interreligious cooperation remains understudied.

Discussion and outlook for further research

After reviewing literature on each dimension of interreligious peace, what do we know about religious determinants of interreligious peace? Table 7 summarizes our main results. We find that religious ideas foster or hinder interreligious peace depending on their content. The current literature on interreligious physical violence, hostile attitudes as well as interreligious trust mainly tests religious ideas that hinder interreligious peace. In contrast, literature on interreligious cooperation reports evidence for religious ideas fostering peace. In fact, calls for violence by religious leaders, fundamentalist convictions, or belief in a (monitoring) God hinder interreligious peace while reminders of norms making outgroup prosociality imperative seem to foster interreligious peace. Theoretical works assume some ideas, like forgiveness, to be key to positive interreligious relations (e.g., Appleby, 2000; Auerbach, 2005), yet their impact on interreligious peace has not been tested. Thus, while it seems crucial to promote inclusive religious ideas and at the same time counteract more problematic, exclusive ideas, studies testing ideas with potentially positive effects are needed as they may not always have expected positive effects (Hoffmann, 2022).

Table 7. Multidimensional impact of religion on interreligious peace

Religious identities are the focus of much of the reviewed literature. Our review finds that “dangerous demographics” as well as strongly identifying with the own religious group can negatively impact intergroup biases, and thereby matter for interreligious peace. However, contextual factors (such as political, economic, and historical conditions) seem crucial in shaping how religious identities matter for interreligious relations. Further, research on interreligious trust and cooperation highlights that preferences for the ingroup do not necessarily go together with outgroup derogation and that salience of identities as well as intersectionality must be considered. Moreover, demography affects opportunities for positive interreligious contact which has positive effects on interreligious relations. Thus, future studies should disentangle under what conditions religious identities pose a risk to peace in a more nuanced way, for example, by combining cross-country data with comparative case-study approaches. For example, theoretical claims that changing religious demography could cause unrest because threat perceptions and motives for acts of violence may arise especially on the part of the shrinking group (e.g., Seul, 1999), remain untested.

Results on the influence of religious practices on interreligious peace are mixed. In light of inconclusive results and insignificant effects of individuals' participation in religious practices on their attitudes or behaviors pertaining to interreligious peace, it does not seem promising to further test these associations. Rather, participation in religious practices may function as a signal to others, for example, of trustworthiness (Ruffle and Sosis, 2007; Hall et al., 2015), but also of a strong ingroup identity or exclusive values (Rink and Sharma, 2018). Additional research must investigate the generalizability of existing findings by replicating these studies in more contexts and evaluate under what conditions religious practices enhance or counteract interreligious peace. A recent qualitative study on Ethiopia, for example, suggests that spatial components of religious practices can play a role in dynamics of competition between religious groups (Østebø, 2023).

Studies on the impact of religious actors and institutions on interreligious peace find that religious institutions ensuring power sharing or equal treatment of religions may prevent interreligious violence whereas religious competition may fuel religious conflict. However, quantitative evidence is scarce. Research focusing on the role of religious actors and institutions in interreligious physical violence could be broadened to investigate further elements of interreligious peace. Additionally, dynamic debates on which types of institutions may best serve diverse societies can inspire further research. For instance, religion–state relations governed by institutions guaranteeing freedom of religion or belief (FORB) are theoretically considered important to enable interreligious peace (Joustra, 2020; Stewart et al., 2020). Indeed, a qualitative study on Myanmar found that violent mobs in Myanmar used claims of mosques being “illegal” as justifications to attack places of worship (Kyaw, 2021). However, the relation between FORB and interreligious peace still warrants quantitative investigations. Moreover, the way religious institutions are organized could affect interreligious peace. For example, strictly hierarchical religious organizations may be less prone to interreligious violence because they have more control over their believers and lower-ranking clergy (cf. Basedau, 2016). Regarding the role of religious leaders for interreligious peace, examining theoretical claims regarding relations between religious leaders and state institutions could further debates (De Juan, 2008). Similarly, empirically studying the assumed effectiveness of religious leaders' rhetoric in promoting interreligious peace (cf. Sharma, 2017) could inform research on interreligious peace.

Our conclusions are limited by the selection criteria for literature included in this review. While criteria are necessary to delimit our search, they restrict our understanding of what is known about interreligious peace. The main limitation is that our review focuses on quantitative literature which implies that evidence from relevant qualitative research is not considered (e.g., Østebø, 2023). Further, we restrict our review to religious determinants of interreligious peace. Yet, there are other important factors like economic welfare (e.g., Mousseau, 2011) affecting interreligious peace. Additionally, we focus on literature published after 2000. Yet, the small number of reviewed studies published prior to 2010 suggests that it is unlikely that we exclude substantial research on this criterium. Finally, we do not review studies focusing on the impact of religion on conflict and peace more generally. Thus, our review is a first step in a greater endeavor to learn about the impact of religion on relational peace more generally.

We would like to emphasize several aspects that should be considered for further research on the link between religion and interreligious peace. First, there is a need for conceptual clarity. For example, “religion” was often not specified and, if specified, sometimes unfittingly—and many times inconsistently—operationalized (see also Basedau et al., 2018). In many studies, religiosity is used as an explanatory variable. However, religiosity is sometimes measured by accounting for theological, ritual, experiential, and consequential dimensions (e.g., Chuah et al., 2014) and other times as the strength of identification with a religion (e.g., Xia et al., 2021). In the absence of differentiated hypotheses on the effect of various aspects of religiosity on interreligious peace, a lack of consistency of definitions makes comparisons across studies difficult.

Second, research findings should be replicated in different contexts to further understand their generalizability. The majority of studies reviewed focus on a single country. Hence, we do not know whether theories also hold in other contexts. Furthermore, especially the literature on interreligious physical violence, hostile attitudes, and threat perceptions relies on cross-country and survey designs. For these literatures, we identify a need for causal studies going beyond correlational designs.

Third, religion impacts interreligious peace through more than religious identities. Even though religion is multidimensional, current research mainly focuses on the role of religious identities. While existing studies highlight aspects of religious identities that hinder peace, conditions under which religious groups live peacefully side-by-side without identities becoming contentious need further study. Moreover, the role of religious actors and institutions may foreground ways religion fosters interreligious peace. Similarly, different religious ideas should be more vigorously investigated to learn about the ambiguity of their effect on interreligious peace.

Finally, it remains a challenge to better integrate different strands of literature and piece together a more cohesive literature. Making claims about interreligious peace requires drawing on various literatures to reflect its multidimensionality. Hereby, it will be important to investigate the interrelation of its dimensions. Existing research suggests a web of linkages between the dimensions of interreligious peace proposed in our definition. Threat perceptions may be an important determinant of hostile attitudes (Kanas et al., 2015), while trust seems to increase cooperation (Balliet and Van Lange, 2013; Preston and Ritter, 2013). In fact, previous violence and interreligious tensions may foment a perceived need for more cooperation and exchange between religious groups (Vüllers et al., 2015). While this is initial evidence for the interrelation of dimensions of interreligious peace, a more in-depth investigation is warranted.