"Without population structure, cooperation based on repetition is unstable," Garcia explains one of the main findings. This is especially true for humans, where repetition occurs regularly and who live in fluid, but not totally unstructured populations. A pinch of population structure helps a lot if repetition is present. "Therefore, the recipe for human cooperation might be: a bit of structure and a lot of repetition," says Julian Garcia. This phenomenon results in a high average level of cooperation. - Science Daily- Does Cooperation Require Both Reciprocity and Alike Neighbors?