Differences of mean pay-offs of cooperators and defectors and assortment in the sexual reproduction model. (a, c) In blue areas cooperators receive greater average pay-offs and in red areas defectors receive greater average pay-offs. (b, d) Blue areas indicate high positive assortment and red values indicate low positive assortment cooperators. (a, b) Scenario without selection (s = 0). (c, d) Scenarios with selection (s = 0.5). Note that results are shown only for cost-benefit ratios up to 0.6.