A comparative anthropological perspective reveals not only that some human societies do not engage in war, but also that peaceful social systems exist. Peace systems are defined as clusters of neighbouring societies that do not make war with each other. The mere existence of peace systems is important because it demonstrates that creating peaceful intergroup relationships is possible whether the social units are tribal societies, nations, or actors within a regional system. Peace systems have received scant scientific attention despite holding potentially useful knowledge and principles about how to successfully cooperate to keep the peace. Thus, the mechanisms through which peace systems maintain peaceful relationships are largely unknown. It is also unknown to what degree peace systems may differ from other types of social systems. This study shows that certain factors hypothesised to contribute to intergroup peace are more developed within peace systems than elsewhere. A sample consisting of peace systems scored significantly higher than a comparison group regarding overarching common identity; positive social interconnectedness; interdependence; nonwarring values and norms; non-warring myths, rituals, and symbols; and peace leadership. Additionally, a machine learning analysis found non-warring norms, rituals, and values to have the greatest relative importance for a peace system outcome. These results have policy implications for how to promote and sustain peace, cohesion, and cooperation among neighbouring societies in various social contexts, including among nations. For example, the purposeful promotion of peace system features may facilitate the international cooperation necessary to address interwoven global challenges such as global pandemics, oceanic pollution, loss of biodiversity, nuclear proliferation, and climate change.