To merge or not: The onto- and phylogenetic origin of co-representation

Co-representation during joint action is a hallmark of human cooperation, but its origin poses a puzzle: It apparently emerges late in ontogeny (around the age of four) suggesting it is cognitively demanding, but it has also been demonstrated in several nonhuman primate species. We reassessed the ontogeny of co-representation in 2-4 years old human children with a language-free task developed for nonhuman primates. Co-representation was already present and strongest in the youngest age class, and not constrained by Theory of Mind and inhibitory control. Together with the primate results, this suggests co-representation emerges spontaneously and automatically, and that not co-representation per se, but the flexibility to adjust when to merge and when not is the fundamental challenge for cooperation success. We show that human children and the most cooperative primates, the cooperatively breeding marmosets, rely on coordination smoothers (marmosets: mutual gaze; children: mutual gaze and communication) to achieve that.