Humans distinguish between we-groups and they-groups, such as relatives versus strangers and higher-ups versus lower-downs, thereby creating crucial preconditions for favouring their own groups while discriminating against others. Reported here is the finding that the extent of differentiation between us and them varies along latitude rather than longitude. In geographically isolated preindustrial societies, intergroup differentiation already peaked at the equator and tapered off towards the poles, while being negligibly related to longitude (observation study 1). Contemporary societies have evolved even stronger latitudinal gradients of intergroup differentiation (survey study 2 around 1970) and discrimination (mixed-method study 3 around 2010). The geography of contemporary differentiation and discrimination can be partially predicted by tropical climate stress (warm winters, hot summers and irregular rainfall), largely mediated by the interplay of pathogen stress and agricultural subsistence (explanatory study 4). The findings accumulate into an index of intergroup discrimination by inhabitants of 222 countries (integrative study 5).
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As indicated in the Methods, all data are available for visual inspection (Supplementary Information) and empirical analysis. SPSS data files for preindustrial and contemporary societies can be downloaded from https://hdl.handle.net/10411/YXI7WH. The author is prepared to provide clarifications if needed.
The SPSS analysis scripts used in studies 1–5 are provided in the Supplementary Methods. The author is prepared to provide clarifications if needed.
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Helpful comments on drafts of this article were received from L. G. Conway, S. Daan, C. K. W. De Dreu, H. C. Santos, W. Scholl and P. A. M. Van Lange. The author received no specific funding for this work.
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Extended Data Fig. 1 Joint effects of pathogen stress and agricultural subsistence on gender discrimination in 157 contemporary societies (R2 = .72).
Horizontally viewed, both slopes tell that higher pathogen stress increases gender discrimination, albeit less so in areas with much agriculture (B(153) = .29, p < .001, CI = .16 to .42 for the upper slope) than in areas with little agriculture (B(153) = .65, p < .001, CI = .49 to .82 for the lower slope). Vertically viewed, both gaps between slopes tell that more agriculture increases gender discrimination, albeit less so in areas with high pathogen stress (B(153) = .36, p < .001, CI = .22 to .49 for the right gap) than in areas with low pathogen stress (B(153) = .72, p < .001, CI = .56 to .89 for the left gap).
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Van de Vliert, E. The global ecology of differentiation between us and them. Nat Hum Behav 4, 270–278 (2020). https://doi.org/10.1038/s41562-019-0783-3