In discussions of various alternative voting methods, it is often claimed that one voting method or another is “vulnerable” or “susceptible” to tactical voting. But such rhetoric is often misleading.
To illustrate that, consider the “Random Ballot” method, with which the election is decided based on a randomly chosen ballot. This has the academically interesting property that it makes a tactical vote equivalent to a sincere vote. That is, it is completely invulnerable to strategy. However, it is clearly an extremely undemocratic system.
To illustrate this concept, consider the following simple graph of hypothetical voting method performance (i.e. “democratic-ness”). The X axis represents the ratio of tactical voters. On the left, 100% of voters are sincere. On the right, 100% are tactical. The blue line for Random Ballot is flat, which demonstrates that its performance is unaffected by tactical voting. The red line is for a hypothetical “normal” voting method, whose vulnerability to strategy is apparent from its downward slope.
This graph reveals how misleading it would be to claim that Normal Method is more vulnerable to tactical voting than Random Ballot. In fact, Normal Method is better with 100% strategic voters than Random Ballot is with 100% sincere voters.