  Recent theory shows that extortioners taking advantage of the
zero-determinant (ZD) strategy can unilaterally claim an unfair share of the
payoffs in the Iterated Prisoner's Dilemma. It is thus suggested that against a
fixed extortioner, any adapting co-player should be subdued with full
cooperation as their best response. In contrast, recent experiments demonstrate
that human players often choose not to accede to extortion out of concern for
fairness, actually causing extortioners to suffer more loss than themselves. In
light of this, here we reveal fair-minded strategies that are unbending to
extortion such that any payoff-maximizing extortioner ultimately will concede
in their own interest by offering a fair split in head-to-head matches. We find
and characterize multiple general classes of such unbending strategies,
including generous zero-determinant strategies and Win-Stay, Lose-Shift as
particular examples. When against fixed unbending players, extortioners are
forced with consequentially increasing losses whenever intending to demand more
unfair share. Our analysis also pivots to the importance of payoff structure in
determining the superiority of zero-determinant strategies and in particular
their extortion ability. We show that an extortionate ZD player can be even
outperformed by, for example, Win-Stay Lose-Shift, if the total payoff of
unilateral cooperation is smaller than that of mutual defection. Unbending
strategies can be used to outlearn evolutionary extortioners and catalyze the
evolution of Tit-for-Tat-like strategies out of ZD players. Our work has
implications for promoting fairness and resisting extortion so as to uphold a
just and cooperative society.
