Altruism in populations at the extinction transition

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We study the evolution of cooperation as a birth-death process in spatially extended populations. The benefit from the altruistic behavior of a cooperator is implemented by decreasing the death rate of its direct neighbours. The cost of cooperation is the increase of a cooperator's death rate proportional to the number of its neighbors. For any benefit-cost ratio above 1, the stable stationary concentrations of agents pass through four regimes as the baseline death rate p increases: (i) defection only, (ii) coexistence, (iii) cooperation only, (iv) extinction. We investigate the transitions between these regimes. Qualitatively identical phase diagrams are obtained for populations on square lattices and in pair approximation. Spatial correlations with nearest neighbours only are thus sufficient for sustained cooperation. These results are published as Klemm & Khalil, Physical Review Research 2, 013374 (2020).