It is not fully understood why we cooperate with strangers on adaily basis. In an increasingly global world, where interactionnetworks and relationships between individuals are becomingmore complex, different hypotheses have been put forward toexplain the foundations of human cooperation on a large scale andto account for the true motivations that are behind this phenom-enon. In this context, population structure has been suggested tofoster cooperation in social dilemmas, but theoretical studies ofthis mechanism have yielded contradictory results so far; addition-ally, the issue lacks a proper experimental test in large systems.We have performed the largest experiments to date with humansplaying a spatial Prisoner’s Dilemma on a lattice and a scale-freenetwork (1,229 subjects). We observed that the level of coopera-tion reached in both networks is the same, comparable with thelevel of cooperation of smaller networks or unstructured popula-tions. We have also found that subjects respond to the coopera-tion that they observe in a reciprocal manner, being more likely tocooperate if, in the previous round, many of their neighbors andthemselves did so, which implies that humans do not considerneighbors’payoffs when making their decisions in this dilemmabut only their actions. Our results, which are in agreement withrecent theoretical predictions based on this behavioral rule, sug-gest that population structure has little relevance as a cooperationpromoter or inhibitor among humans.