 Let's say there are two major parties in your city. Imagine that you like one better than the other, so we're going to call them the good party and the bad party. Here's the good news. The good party has a 10-point advantage in your city and wins most elections. The bad news is that your city uses plurality voting. And the upcoming election for mayor has three candidates. And you really like the independent candidate this time. Even though he's your favorite choice, you know you can't vote for him. If he takes too many votes away from the good candidate, that would cause the worst candidate to win. Wouldn't it be great if your city used a voting system that didn't have this problem? How about instant runoff voting? Then you can vote for your favorite. With instant runoff, when your ideal candidate gets eliminated, your vote moves over to help the good candidate win. That's great, right? But what happens when your ideal candidate does even better? What if he actually beats your second choice and your second choice gets eliminated? Unfortunately, that puts us completely at the mercy of these voters that we have no control over. Those who voted for the good candidate. And who did they put for their second choice? It would be nice if they had all put our ideal candidate second, but they didn't. It takes only one-fifth of them putting the bad candidate second for him to win this election. What happened here? Doesn't instant runoff eliminate the spoiler problem? It turns out that putting your favorite first in instant runoff is only safe when your candidate is very strong or has no chance at all. In between, there's a good chance he will have to face your least favorite candidate before he's ready. In this way, putting your favorite candidate first can cause your least favorite candidate to win. Ouch. In fact, if you and some of your fellow idealist supporters had forgotten to show up, then your favorite candidate would have been eliminated, but your second favorite candidate would have won. Does this problem happen in every election? No. This does not happen in every scenario. It doesn't happen this way when the third candidate pulls supporters from both of the major candidates equally, but it does happen whenever he pulls mostly from one and not the other. Hmm. To me, that sounds like a lot of the three-way elections in America. What about later no harm? Instant runoff advocates are fond of quoting the later no harm principle. Adding a second choice, third choice, or more to your ranking will never hurt your first choice. Guess what? This is completely true. But note how carefully it's worded. It never says that it's safe to put your favorite candidate for your first choice. With instant runoff voting, your first choice has to be a safe candidate. Someone who can beat your least favorite. Putting your favorite candidate first might cause your second choice to get eliminated and your third choice to win. So later no harm is great, but it's not a promise that it's safe to put your favorite first. Be careful voting for your favorite with instant runoff voting. You might eliminate the strongest candidates and cause your least favorite candidate to win. If you want a voting system that avoids this problem, then you care about the favorite betrayal criterion. The favorite betrayal criterion says, voters can never get a worse result by expressing the maximum support for their favorite candidate. Approval voting is one system that passes the favorite betrayal criterion. Look for more information about this criterion and about approval voting in a later video.