In-Group---Out-Group Trust: Origins and Consequences

Abstract

Why do societies differ in the extent to which they trust beyond the in-group? This paper tests the hypothesis that the ecological and institutional demands of transhumant pastoralism shaped long-run variation in the scope of trust. Combining global data from 104 countries, individual-level surveys, and within-country evidence from Kyrgyzstan and Sudan, we show that ancestral reliance on transhumant pastoralism predicts higher trust in in-group members relative to out-group members. We then uncover the mechanism using a newly assembled Atlas of Transhumance Routes: the effect is significantly stronger along flatter, more ''horizontal'' routes that historically generated recurrent encounters with other pastoralists under weak external enforcement. These routes intersect more ethnic boundaries, and the intensity of such encounters amplifies the transhumance-trust relationship, especially where interactions involved competing pastoral groups. A natural experiment, the 2011 partition of Sudan, which abruptly obstructed long-standing migration corridors, shows that conflict rises sharply along historical routes once mobility becomes constrained. Finally, we show that a narrower scope of trust constrains organizational hierarchies: firms located in historically transhumant regions are more likely to be manager-owned, less likely to be incorporated or have dispersed ownership, and employ fewer workers. Together, these findings reveal how the ecological logic of mobile pastoralism forged enduring trust boundaries that continue to shape patterns of cooperation, conflict, and economic organization today.