America’s two-party system is widely hated. Very few Americans think the two major parties do an adequate job representing us, and most say more parties are needed. But when it comes time to vote, very few of us actually vote for third-party candidates. Often, this is explained as either a failure of will (we’d have third parties if more people would just vote for them), or a conspiracy (the political and media establishments suppress third-party candidates and ideas).

And it’s not that those things aren’t true. But there’s a much simpler explanation, and it’s the very basic rule governing almost every single one of our elections: Only one person can win. If you’re American, that probably sounds utterly reasonable: what the hell other kinds of elections even are there? But the answer is: lots. Winner-take-all elections (also called plurality voting, or “first past the post”) are actually a practice that most advanced democracies left behind long ago — and they’re what keep us from having more political options.

Even if you’re not sold on the need for more parties in the US, though, scratch the surface of “only one person can win” a little and you start to see how it actually produces perverse results within the two-party system as well. It’s a big part of why the political parties have moved farther apart from each other, and it leaves about half of the country without any political representation at all. Watch the video above to see how.