The concurrency constraint: why cultural traits spread and interstellar signals do not

Sebastian Fajardo

Leiden University

Cultural evolution asks why a trait appears here and not there, while SETI asks why we observe silence in the universe. Both questions are usually answered by appealing to the mechanisms of transfer such as adoption cost, effective distance, signal power, or receiver sensitivity. In this talk I argue that both are, at bottom, questions about temporal overlap, and that the better-evidenced case—cultural diffusion in the human past—can be used to interrogate the less well-evidenced one. To do so, I propose using Petri nets, a modelling tool introduced to cultural evolution to treat cultural processes as concurrent systems whose complexity can be measured structurally and behaviourally. I formalise cultural transmission as a stochastic Petri net in which tokens are communities and trait transfer is a transition, occurring only when a demonstrating community and a naive one co-occur. Two results follow from the net's topology. First, the concurrency constraint is intrinsically weaker in the cultural domain. Second, the by-products of practice that keep circulating after their source is gone partially decouple transmission from co-occurrence. If residue carries most cultural transfer, a cultural-evolutionary perspective supports prioritising the search for artefacts over interstellar signals, not because artefacts are more likely, but because they are not window-limited. Finally, I will argue that the Petri net framework yields a domain-neutral decomposition of transfer probability into co-occurrence, encounter and adoption, allowing these three elasticities to be measured in an interstellar counterpart model.