This is a story about the swing voter. The voter who, days before the election, doesn’t know if she wants to vote for Donald Trump or Hillary Clinton — or, hell, maybe Jill Stein or Gary Johnson.

You're going to hear a lot about these mythical middle-ground people in the runup to the election. You can just search "swing voter" to see how common this narrative is:

This is because many pundits think swing voters decide elections. They think swing voters — or "floating voters" — are large in number and just sitting in the middle, waiting to be convinced by one party or the other to vote for them. So this is the picture in their heads:

But increasingly, this picture is wrong. As the parties polarize, it’s becoming much easier to for Americans to choose sides, even if they don’t identify as Republican or Democrat. So when they go to vote, they act like partisans.

Back when there were more voters who might change their minds, campaigns were based on persuasion, and persuasion pulls you toward the middle. But when everyone’s mind is already made up, that means the way to win an election is to rile up people on your own team.

We know swing voters are going extinct because of a political scientist named Corwin Smidt

A few years ago, Smidt, who works at Michigan State University, was trying to reconcile two things.

One was that there were more and more people identifying as independents, rather than Democrats or Republicans.

That should've meant that there were a huge number of people who were willing to change their minds, since they weren't partisan.

But Smidt also noticed that over this time period, the presidential election maps didn't change much. It was “much more rigid."

One way to reconcile these two things was to believe that independents were no longer participating, which means there were fewer nonpartisan voters.

But Smidt had another question: Are these self-labeled nonpartisans actually willing to vote for Republicans one election and Democrats in another?

So he decided to study historical surveys that asks Americans about how they voted. He came up with four groups for the respondents:

He did this for every year going back 50 years.

What he found was simple, though shocking: Even though more and more people don’t align with a party, they still consistently behave like reliable partisans and repeatedly vote for the same party.

In other words, swing voters are dying.

Why are swing voters going extinct?

Most voters look at where the parties stand on the issues, and then they pick one that best reflects their own views. But if you don’t pay enough attention to tell the parties apart, then you won’t know which party you agree with. That makes you more likely to swing between parties.

So if we redraw the picture at the top, it would look like this. Notice how similar the two sides look to her:

And it used to be common for people with less political knowledge to switch the party they supported from election to election — whether or not they voted.

But over time, this party switching has become far less common:

This is because now even low-information voters can tell the two parties apart:

Even low-information voters say there’s an important difference between Republicans and Democrats

In 1960, both partisans and nonpartisans were much less likely to say there was a big difference between Republicans and Democrats.

Now almost all strong partisans would say yes — and even half of nonpartisans would say yes.

In short, voters are more confident that they know exactly what they’re voting for when they look at the “D” or the “R” next to a candidate’s name — no matter how partisan they are.

And it's not just about partisanship.

A modern-day person with low political awareness can tell the parties apart just as well as someone who had high awareness in 1968:

How is it that swing voters can now tell the parties apart? Polarization.

It's not that the people have changed. Rather, it's the parties.

Let's go back to the drawing at the top of the story. Here's where we were in the 1950s, where a low-information voter would look at the parties and not be able to tell the difference.

But over the past 50 years, the two parties have evolved very distinct platforms.

In the 1968 election, neither Republican Richard Nixon nor Democrat Hubert Humphrey articulated clear views on what he would in Vietnam. "This ambivalence and deliberate obfuscating can create opportunity to win voters over," Smidt said.

Before Ronald Reagan, the Republican Party didn't have an anti-abortion position.

At the 1996 State of the Union, Bill Clinton said, "The era of big government is over."

It was a lot harder to pin down each party to specific issues, especially for those who weren't paying close attention. But now the party lines are clear on abortion, taxes, climate change, health care, same-sex marriage — the list goes on. That's why when we look at how much Republicans and Democrats agree in the House and Senate, we see more and more polarization:

You could argue that voters clearly knowing which side they’re on is a good thing. But in the 1954 study "Voting: A Study of Opinion Formation in a Presidential Campaign," political scientists argued that what we need from an electorate is diversity — in other words, we can't just have rigid voters with ideal views, but also detached voters with rational views. Those are the people who allow our political system to more efficiently get things done.

As Smidt writes, "Even the 'least admirable' voters enrich our democratic political system by providing it with the flexibility and indifference that it needs."

If everyone is a reliable partisan, then there isn't incentive to be in the middle ground. And we can't get things done.

If more and more voters are either on your side or the opponent’s side, that means winning elections is simple: You have to get your people more excited, and get the other side less excited.

This means that candidates have little incentive to talk to people in the middle — people who don't make consistent partisan decisions in the voting booth.

Just think about the 2016 election in this framework: Voters in Utah, who are traditionally Republican, don't like Donald Trump. But because the party positions are so polarized, it's too far a jump for them to support a Democrat like Hillary Clinton. So as Smidt points out, many would rather jump to a third party candidate.

Trump was supposed to be the most nontraditional candidate in modern American politics — someone who was shaking up the electorate. But on November 8, when you look at the election map, it'll probably look a lot like past election maps. Maybe a few states will switch over, but most of America will likely have voted the same way.

The president-elect will have won on the backs of riled up partisans, and people would be swing voters — except they’re not because they picked a side long ago.

Clarification: I initially called Smidt’s research an experiment, but it’s more so a study of historical data, which I’ve clarified in the piece.