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. 2014 May 29;5:3976. doi: 10.1038/ncomms4976

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The graph shows the effects of of human cooperation on the payoffs of ZD strategies (a,b) and on the human subjects’ payoffs (c,d). The horizontal axis shows the fraction of rounds in which the human players cooperated. Coloured dots represent the outcome of the experiment, whereas the dashed line depicts the linear regression curve based on a least squares analysis. Human cooperation had a strongly positive impact on the co-player’s payoff, and a weakly positive impact on the own payoff. Thus withholding cooperation punishes extortion.