In 1990, shotguns and M-16s were adopted into Enga warfare, setting off some 15 years of devastation as youths (similar to 17 to 28) took charge of interclan warfare. In response, people called on elder leaders to adapt customary institutions to restore peace; subsequently, war deaths and the frequency of war declined radically. Data from precolonial warfare, 501 recent wars, and 129 customary court sessions allow us to consider (i) the principles and values behind customary institutions for peace, (ii) their effectiveness, (iii) how they interact with and compare to state institutions of today, and (iv) how such institutions might have shaped our human behavioral repertoire to make life in state societies possible.