 The requirement of strategy-proofness has been challenged too. One line of argument is that, even when there exist strategic incentives in the technical sense of the Gibbard-Satterthwaite theorem, individuals will not necessarily act on them. They would require detailed information about others’ preferences and enough computational power to figure out what the optimal strategically modified preferences would be. Neither requirement is generally met. Bartholdi, Tovey, and Trick (1989) showed that, due to computational complexity, some social choice rules are resistant to strategic manipulation: it may be an NP-hard problem for a voter to determine how to vote strategically. In this vein, Harrison and McDaniel (2008) provide experimental evidence suggesting that the ‘Kemeny rule’, an extension of pairwise majority voting designed to avoid Condorcet cycles, is ‘behaviourally incentive-compatible’: i.e., strategic manipulation is computationally hard.