This chapter explores co-evolutionism, focusing on the work of Peter Richerson and Robert Boyd, who posit that rules and institutions evolve through the interaction of biological and cultural processes. They propose a Darwinian model of cultural evolution, where natural selection acts on cultural variation, directly and indirectly, defining culture as information acquired via imitation or social learning that affects an individual’s phenotype. Richerson and Boyd identify three evolutionary forces: random forces, decision-making forces, and natural selection directly on cultural variation. Their model of cultural selection demonstrates that non-parentally transmitted cultural variants can influence individuals significantly, potentially even reducing Darwinian genetic fitness. However, genetic selection may favour such transmission due to its benefits in information acquisition, especially in variable environments. Ultimately, they advocate for a synthetic theory of human behaviour to reconcile biology and social sciences, arguing that understanding cultural transmission is crucial for explaining human evolution and behaviour beyond traditional Darwinian predictions. Richerson and Boyd’s co-evolutionary framework offers a relevant perspective for understanding the interplay of biological and cultural factors in the evolution of rules and institutions, including law.