Humans adapt to social diversity over time Miguel R. Ramosa,b,1, Matthew R. Bennettc, Douglas S. Masseyd,1, and Miles Hewstonea,e aDepartment of Experimental Psychology, University of Oxford, Oxford OX2 6AE, United Kingdom; bCentro de Investigação e de Intervenção Social, Instituto Universitário de Lisboa (ISCTE-IUL), 1649-026 Lisboa, Portugal; cDepartment of Social Policy, Sociology and Criminology, University of Birmingham, Birmingham B15 2TT, United Kingdom; dOffice of Population Research, Princeton University, Princeton, NJ 08544; and eSchool of Psychology, University of Newcastle, Callaghan, NSW 2308, Australia Contributed by Douglas S. Massey, March 1, 2019 (sent for review November 2, 2018; reviewed by Oliver Christ, Kathleen Mullan Harris, and Mary C. Waters) Humans have evolved cognitive processes favoring homogeneity, (21) recognizes that humans share an impulse to engage in contact stability, and structure. These processes are, however, incompatible with other people, even those in outgroups. Indeed, biological and with a socially diverse world, raising wide academic and political cultural anthropology contend that humans have evolved and concern about the future of modern societies. With data comprising fared better than other species because contact with outgroups 22 y of religious diversity worldwide, we show across multiple brings a variety of benefits that cannot be attained by intragroup surveys that humans are inclined to react negatively to threats to interactions. There is, for example, a biological advantage to homogeneity (i.e., changes in diversity are associated with lower gaining genetic variability through new mating opportunities (22) self-reported quality of life, explained by a decrease in trust in and intergroup contact allows individuals access to more diverse others) in the short term. However, these negative outcomes are resources and knowledge (23). Archaeological and ethnographic compensated in the long term by the beneficial influence of evidence thus suggests that the benefits of trust and collaboration intergroup contact, which alleviates initial negative influences. largely outweigh the potential costs of intergroup conflict, con- This research advances knowledge that can foster peaceful coex- tributing to the proliferation of group-beneficial behaviors over istence in a new era defined by globalization and a socially time (5, 24). Indeed, research has demonstrated through a large diverse future. body of work and meta-analyses that contact with outgroups re- liably improves intergroup relations (25, 26). Studies indicate that social diversity | trust | intergroup contact | well-being | health diverse contexts create greater opportunities for intergroup con- tact, which in turn promotes trust and social cohesion (27). lobal modernization has dramatically changed the demo- On balance, although a strong motivation for homophilous Ggraphic composition of most countries (1–3). Societies are affiliation may be critical for within-group vitality and collabo- in constant flux and current levels of intercultural exchange are ration, it is more suited to the monocultural composition of transforming social ecosystems (4). Pessimistic appraisals about ancestral social structures and less compatible with the social the potential effects of these changes have dominated recent and diversity typical of modern societies. We argue, however, that critical geopolitical events (e.g., the election of Donald Trump as this incompatibility will dissipate over time given that, as socie- president in the United States, Brexit in the United Kingdom, ties become more diverse, individuals steadily reorient them- and the refugee crisis globally), but it is not yet known how living selves by adopting outgroup-focused cognition and behaviors (1). ’ in a socially diverse world affects the quality of people s lives. Within this framework, we examine how both negative (i.e., re- Models of human evolution applied to social diversity (1) sup- duced trust stemming from diversity) and positive (i.e., increased port the notion that adaptation to a socially diverse context may be problematic and lead to negative outcomes. According to these Significance models, the human brain evolved to sustain motivated cognition and behavior relevant to ingroup survival and cooperation, and to defend against potential threats from unknown outgroups (5). Changes in social diversity constitute a key factor shaping to- ’ Humans are predisposed to distinguish ingroups from outgroups day s world, yet scholarly work about the consequences of and this dichotomy is adaptive given that survival is contingent diversity has been marked by a critical lack of consensus. To upon cooperation and reciprocity from other ingroup members (6, address this concern, we propose a multidisciplinary approach 7). Perhaps to facilitate this dichotomization, humans have evolved where psychological, sociological, and evolutionary perspec- a preference for homogeneity and stability (8), as well as being with tives are integrated to provide an account of how individuals similar others (9). Moreover, outgroups are approached with a adapt to changes in social diversity. With an analysis of 22 y of degree of uncertainty (10), as unknown others could be friends or worldwide data, our results suggest that humans are initially foes, and caution in new encounters could dictate one’s survival. inclined to react negatively to threats to homogeneity, but that This reasoning is substantiated by influential work in the social these negative effects are compensated in the long term by the sciences showing that interpersonal trust and social cohesion are beneficial effects of intergroup contact. Our findings advance knowledge and inform political debate about one of the de- lower in ethnically heterogeneous communities (11–13). Sub- fining challenges of modern societies. sequent work across multiple disciplines expanded on these find- ings by revealing that social diversity is associated with conflict (14) Author contributions: M.R.R., M.R.B., D.S.M., and M.H. designed research; M.R.R. per- and may have negative implications for economic growth (15) and formed research; M.R.R. analyzed data; and M.R.R., M.R.B., D.S.M., and M.H. wrote the public goods provision (16). The mechanism hypothesized to paper. underlie these outcomes is that diversity erodes social cohesion Reviewers: O.C., Fernuni Hagen; K.M.H., The University of North Carolina at Chapel Hill; and trust in others (17). Meta-analyses in the field of psychol- and M.C.W., Harvard University. ogy substantiate this reasoning by demonstrating that, at least Conflict of interest statement: M.H. and O.C. are coauthors on several recent articles. initially, intergroup interactions can exacerbate intergroup Published under the PNAS license. bias, producing heightened stress, anxiety, and outgroup See Commentary on page 12131. avoidance (18). 1To whom correspondence may be addressed. Email: [email protected] or Nonetheless, negative effects of diversity on trust have been [email protected]. contested by recent literature reviews (19, 20) and, despite the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. foregoing predispositions, it is known that the extension of 1073/pnas.1818884116/-/DCSupplemental. cooperation to outgroups is hardly rare. Research on xenophilia Published online May 9, 2019. 12244–12249 | PNAS | June 18, 2019 | vol. 116 | no. 25 www.pnas.org/cgi/doi/10.1073/pnas.1818884116 Downloaded by guest on September 24, 2021 trust through intergroup contact) effects operate to push or pull We then used these three quality-of-life indicators to compose a outcomes in either direction. quality-of-life latent variable that was incorporated at higher Using worldwide data, we explore how these processes unfold in analytical levels. This technique permitted modeling individual SEE COMMENTARY the context of growing religious diversity. Changes in religious unobserved heterogeneity due to omitted variables that could be diversity are shaping today’s world and provide a powerful context correlated with changes in key variables over time (35). relevant to the aims of our study. People with different religious As controls, in all surveys we include a range of pertinent in- beliefs often have distinct norms, values, and worldviews that are dividual (e.g., education and religious affiliation) and contextual the product of centuries of cultural evolution and cannot be easily (e.g., country wealth and economic inequality) variables. To changed or negotiated. Religion has historically been one of the achieve an exact replication across surveys, only variables present strongest forces of human division, motivating expressions of in- in all datasets were included (for further details about the sur- tergroup hostility and outright warfare (28). Religion has most veys, control variables, and modeling technique see SI Appendix). recently been at the forefront of public debate and political vio- Available data from the WVS (68 countries, 142 country-years lence ranging from major attacks such as September 11, 2001 from 1981 through 2014, and 160,645 respondents) reveal that in the United States to smaller but still deadly incidents in multiple after 1995 there were short-term fluctuations in religious diversity European countries (e.g., the United Kingdom, France, and around the world until 2000. Thereafter diversity increased Spain). Today, religious diversity is of paramount relevance to every