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Evidence for third-party mediation but not punishment in Mentawai justice


Researchers argue that third parties help sustain human cooperation, yet how they contribute remains unclear, especially in small-scale, politically decentralized societies. Studying justice among Mentawai horticulturalists in Indonesia, we examined evidence for punishment and mediation by third parties. Across a sample of 444 transgressions, we find no evidence of direct third-party punishment. Most victims and aggrieved parties demanded payment, and if a transgressor faced punishment, this was never imposed by third parties. We find little evidence of indirect sanctions by third parties. Nearly 20% of transgressions were followed by no payment, and as predicted by dyadic models of sanctions, payments were less likely when transgressions were among related individuals. Approximately 75% of non-governmental mediators called were third parties, especially shamans and elders, and mediators were called more as cooperation was threatened. Our findings suggest that, among the Mentawai, institutionalized penalties function more to restore dyadic cooperation than to enforce norms.

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Fig. 1: A victim lists his demands.
Fig. 2: The distributions of tulou payments for different categories of transgression.
Fig. 3: Predictors of a tulou payment occurring.
Fig. 4: Predictors of being called to mediate.
Fig. 5: Simulated rates of third-party mediation.

Data availability

All cleaned, anonymized data are available at the OSF project page at

Code availability

All code used in analyses is available at the OSF project page at


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L. Fitouchi, L. Glowacki, J. Henrich, H. Larreguy, S. Mathew, C. Molho, C. von Rueden and members of IAST’s Social Evolution Team shared valuable comments on earlier versions of this manuscript. M. Delfi, B. Sakaliou and G. Sakaliou assisted with fieldwork. R. Henry and J. Tulius assisted with clarifying Mentawai terminology. Data collection by M.S. was funded by a National Science Graduate Research Fellowship, a Sheldon Traveling Fellowship from the Harvard Committee on General Scholarships and a grant from the Mind, Brain and Behavior Initiative at Harvard University. Both authors acknowledge IAST funding from ANR under grant no. ANR-17-EURE-0010 (Investissements d’Avenir programme).

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Authors and Affiliations



M.S. collected data as part of ongoing field research. M.S. and Z.H.G. conceived the study, designed and conducted analyses, and wrote the manuscript.

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Correspondence to Manvir Singh.

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Nature Human Behaviour thanks Lee Cronk, Gerard Persoon, Jeffrey Winking and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data

Extended Data Fig. 1 Predictors of a case being mediated.

The figure shows the estimated log odds (x-axis) for predicting whether a mediator was called (model 3) (n = 199 cases). ‘Same clan status’ is a binary variable capturing whether disputants are in the same clan (1) or different clans (0). Points and error bars are posterior means with 95% credible intervals. The shaded areas and distributions respectively represent 50%, 80%, and 95% of the posterior distributions.

Extended Data Fig. 2 Comparing governmental and non-governmental mediation.

The figure shows the estimated severities of transgressions (x-axis) for which local mediators were called and for which governmental officials were called to mediate (n = 208 cases). Points and error bars are posterior means with 95% credible intervals. The shaded areas and distributions respectively represent 50%, 80%, and 95% of the posterior distributions.

Supplementary information

Supplementary Information

Supplementary Figs. 1 and 2 and Tables 1–10.

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Singh, M., Garfield, Z.H. Evidence for third-party mediation but not punishment in Mentawai justice. Nat Hum Behav 6, 930–940 (2022).

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