Forty percent of all registered voters are independents — the highest figure since Gallup started keeping track half a century ago.

Since 2008, 2.5 million voters have left the two major parties to become independents, which is now a larger group than registered Republicans or Democrats. Let’s address some of the misconceptions about this growing and influential bloc of voters.

1. Independent voters aren’t really independent.

Perhaps the biggest myth about independents is that they are closet partisans or “leaners” who are independent in name only but regularly vote with one party.

True, about half of independents do fit into this category, but the rest are truly independent; their allegiance swings from election to election. They are persuadable, not polarized partisans. A recent Pew Research Center poll puts the number of swing voters this year at 23 percent — almost a quarter of the electorate.

In 2006 independents chose Democratic House candidates over Republicans, 57 percent to 39 percent. But in 2008, Democrats won independent voters by only eight points and lost them by 19 points in 2010.

With that kind of track record, it is impossible to say that independent voters are reliably partisan.

2. Independent voters are less engaged.

In hundreds of interviews with independent voters, I found that they tend to be well informed and care about the political process — even though the two parties have done their best to alienate them through attacks, gridlock and dysfunction.

About two-thirds of them say they are independent because “both parties care more about special interests than about average Americans,” according to a Pew survey.

Independent turnout is typically lower than it is among partisan voters. But in more than half of the country, independents are not permitted to vote in primaries, so they have no say in the candidates selected in the general election. It’s no surprise, then, that they are usually less satisfied with their candidate choices than partisan voters are.

Independents are more turned off than partisan voters by negative campaign ads and are more likely to say they want more substantive discussions from the candidates and the media. Independents take voting seriously but are less moved by partisan appeals.

They care more about the deficit than Democratic voters do, more about the environment than Republicans do, and less about social issues, such as same-sex marriage and abortion, than do voters from either party.

3. Independent voters want a third party.

I found no unanimity: Some of them think we do need a third- or multi-party system and consistently vote for outsider and third-party candidates, while others accept that this is a two-party nation.

The most successful third-party presidential candidacy in the past 100 years was when Teddy Roosevelt ran for a third term as a candidate with the Bull Moose Party in 1912 and won 27 percent of the vote. Roosevelt came in second to Woodrow Wilson and carried half a dozen states, including California, Michigan and Pennsylvania. Ross Perot, running as a Reform Party candidate in 1992, won 19 percent of the vote.

The third-party organization Americans Elect gathered enough signatures to qualify for the ballot in more than half the states, but did not attract a candidate who could generate much interest and has officially suspended its effort.

Many independent voters think it is more realistic to push for open primaries, and campaign finance and congressional redistricting reform that would open up the process to all voters and candidates, than it would be to try to create a competitive third party.

4. Independents are centrists.

Independent voters are more diverse in age, race, gender and income than their Republican and Democratic counterparts. Most independents are socially liberal, fiscally responsible centrists, but some libertarians and far-left progressives also call themselves independents.

Sixty percent of independents say they are not aligned with a party because they agree with the Republicans on some things, such as the economy and national security, and with the Democrats on social issues.

I think of independent voters as falling into four key constituencies: NPR Republicans who are socially moderate and fiscally conservative; America First Democrats who tend to be male and more socially conservative (formerly known as Reagan Democrats); the Facebook generation of voters younger than 35 who lean libertarian on social and economic issues; and Starbucks moms and dads, suburban voters who make up a huge chunk of the electorate and are reliably unpredictable.

Washington and the nation at large may seem polarized, but a majority of voters consider themselves somewhere in between the two parties. These are not just independents, but centrists and moderates who still may be registered with one party, but at times vote for the other. Independents and their views are diverse — they don’t fit into either political party or neatly in the center.

5. Independent voters are disillusioned with President Obama.

In 2008, Barack Obama won 52 percent of independent voters to John McCain’s 44 percent, the largest margin a Democratic presidential candidate has received from independents since 1996.

Almost half of the independents who voted for Obama in 2008 did not vote Democratic in 2004; and independents made up 23 percent of Obama voters, according to a study by Third Way, a centrist think tank.

Because their numbers are growing and the Democratic and Republican parties are shrinking, independent voters, especially those in swing states, hold the key to the 2012 election.

Gallup polling has shown Obama and presumptive GOP nominee Mitt Romney in a virtual tie among independent voters.

The most recent poll had Obama up two points with this group nationwide but also revealed that he has a nine-point lead among independents in 12 battleground states.

According to a recent Washington Post poll, independents favor Obama’s support for same-sex marriage by a margin of 49 to 43 percent, but independents still think the economy is the most important issue and probably will make their decision based on how it is doing in the fall.

Ninety percent of Democrats and Republicans say they plan to vote for their party’s candidate, while a third of independent voters say they are not sure how they will vote. That means you will see a lot more visits by Obama and Romney to battleground states, trying to capture independent swing voters.

Linda Killian, a senior scholar at the Woodrow Wilson Center for Scholars, is the author of “The Swing Vote: The Untapped Power of Independents.” This column was distributed by The Washington Post, where it first appeared.