GEOFF BENNETT: The country's divisions often go beyond disputes over policy, regularly spilling into clashes over identity and culture, and pitting friends and family against one another.

Judy Woodruff explores how that came to be and what it means for our shared future in her latest installment of America at a Crossroads.

CLAIRE JERRY, National Museum of American History: Every president has encountered division of some type, much of it partisan, protests, civil unrest, much of it rooted in those very things Washington was concerned about.

JUDY WOODRUFF: Inside the exhibit on the presidency at the National Museum of American History in Washington, curator Claire Jerry hears echoes of the divisions today in our country's past, starting with our very first president, George Washington.

CLAIRE JERRY: In his farewell address, he said it was really worried about three things for the country.

He was worried about regionalism, partisanship and foreign entanglements, and especially the partisanship issue.

He was not a believer in parties that would take the lead over ideas, and one of the things he says in the address is that the unity of government made us a people, and we should be justifiably proud and committed to that.

CARROLL DOHERTY, Director of Political Research, Pew Research Center: The country is more divided, certainly along partisan lines, than we have seen it.

JUDY WOODRUFF: In our first story, we heard from the Pew Research Center's Carroll Doherty and Jocelyn Kiley about how divided the country has become and how hostile members of both parties now are to the other side.

JOCELYN KILEY, Associate Director of Research, Pew Research Center: I think one way to think about this is that people have internalized partisan identity maybe in a way that we didn't really see, say, three decades ago.

MICHELLE VITALI, Pennsylvania: I do think that things have broken down.

I have neighbors that we sort of wave to each other, and that's the extent of our relationship now.

JUDY WOODRUFF: That's a feeling we have heard from our viewers too, that conversations about current events and politics have become far more divisive and personal.

FABIAN GONZALEZ, Texas: Those items that are in the news today, COVID, immigration, politics, abortion, and the list goes on, I'm not free to speak about any of those things, because I fear the consequence of a conversation I don't feel like I can have.

KARA ELLIS, Ohio: It's really hard, because these are people I care about.

These are -- these are people I'm close to that I -- that I have grown up with, I have lived in the same house with.

The underlying currents between all of us is very tense.

SUDHANSHU MISRA, Massachusetts: I would like to talk about politics with my -- discuss politics with my friends.

I would like to share ideas, exchange notes with them.

But, unfortunately, we are at a dead end, where there is a wall.

LILLIANA MASON, SNF Agora Institute, Johns Hopkins University: Decades ago, we disagreed over things like the role of government or the size of government or what we wanted the government to be doing, and with those types of divisions, we can find a compromise.

JUDY WOODRUFF: Lilliana Mason is a political scientist at Johns Hopkins University who draws on social psychology to try to better understand our political divisions.

LILLIANA MASON: What we're seeing today is, the divide is much more about our feelings about each other.

We are angry at one another.

Democrats and Republicans don't trust one another.

We are more likely to dehumanize people in the other party.

We think that they're a threat to the country.

And these types of feelings are not the kind of thing we can compromise with.

JUDY WOODRUFF: Mason opened her first book, "Uncivil Agreement," with the story of Robbers Cave, a famous social science experiment from the 1950s, when researchers brought fifth grade boys to a summer camp outside Oklahoma City.

The boys, all white, were separated into two teams, one calling itself the Rattlers, the other the Eagles.

They were allowed to bond.

Then, after a week, the groups were introduced to each other.

LILLIANA MASON: And they immediately wanted to start competing.

So they wanted to have baseball games, all kinds of different kinds of competitions to prove that they were the best.

So they started calling each other names.

They accused each other of cheating.

They tried to sabotage each other.

The competition got so intense that, ultimately, they had to stop the experiment because they were throwing rocks and they were becoming violent.

And that experiment was used to talk about the sort of innate nature of humans to form groups, to become proud of the groups that we're in, to want our groups to be better than the people that are not in our group, and, ultimately, to compete against another group, if we feel like they are -- they are threatening the status of our team.

JUDY WOODRUFF: Jumping ahead from George Washington's warning at our founding about the danger of political teams... MAN: It is with pride that I place before this convention for president of the United States The name of Dwight David Eisenhower.

(CHEERING AND APPLAUSE) JUDY WOODRUFF: ... the 1950s, when our political parties were a far more ideological mix than today, with conservative and liberal wings in both camps, and when someone like General Dwight Eisenhower was courted by both parties to run as their standard-bearer.

CLAIRE JERRY: Eventually, he chose the party, but yet was still elected with overwhelming support from the American people.

And that would have been true, I think, regardless of which direction he had gone.

LILLIANA MASON: In 1950, the American Political Science Association actually put out a report saying, we need the parties to be more different, because people don't know which party to vote for because they can't tell the difference between them.

And so they can't make a responsible decision.

And, ultimately, what they suggested was that the two parties should really stand for some very different policy ideas.

LYNDON JOHNSON, Former President of the United States: We must not fail.

Let us close the springs of racial poisoning.

JUDY WOODRUFF: In the 1960s, the passage of the Civil Rights Act and the Voting Rights Act by Democrats helped usher in a major realignment of the parties, with many Black Americans becoming Democrats, as many white Americans opposed to integration left that party.

Layered on top of that broad reorganization along racial lines, the 1980s witnessed the mobilization of the socially conservative Christian right, as well as business interests aligned with Republicans.

And eventually came the rise of partisan talk radio, cable TV news, the Internet and social media, exacerbating the divide along partisan lines.

LILLIANA MASON: And, ultimately, what ended up happening is that our society changed in such a way that our parties started becoming different on their own, not based on the policy preferences, or not only based on policy preferences, but based on what Democrats and Republicans looked like, what kind of religious services they attended, what kind of cultural television shows they watched, where they lived.

And so they started really becoming different from each other in a social way, not just in a sort of policy way.

JUDY WOODRUFF: Lilliana Mason argues that this stacking of identities on top of one another into what she calls a mega-identity has reinforced our basic human instinct for inclusion and exclusion, and that that helps explain the tribal politics we see today.

MICHELLE VITALI: I was a practicing Catholic for most of the years that I lived here.

And I just needed to bow out completely, because I don't understand where this sort of militancy is coming from.

And, in fact, it seems to have been created out of whole cloth in order to get people to show up at the polls, show up at events, show up at March For Life in Washington or whatever the cause may be.

EDWARD JACACK, Ohio: Everything from dating sites, right?

I have been single through a lot of this Trump era.

And the first time on the dating sites, no Trumper, Trumper, no Trumper.

I get it, but probably you and I -- and, by the way, I'm not a Trumper -- but you and I could probably agree upon 70 percent of how society works and the things we go ahead and want.

FABIAN GONZALEZ: Whereas, before, we are Americans, we're going to make us win, and now it's going like, no, it's about this little faction of political idealism, and my side is right, and your side is wrong, and there ain't no middle.

LILLIANA MASON: Not that we have never had partisan animosity.

The difference is that now, because of our sort of progress in terms of civil rights, not just for Black Americans, but for all Americans who have previously been marginalized, including women, is that we have associated the two parties with different sides of that story.

Essentially, the left is now taking the position of, we want a fully egalitarian, pluralistic, multiethnic democracy.

We have never fully had it, but we want to make it happen.

And what Trump has been saying, right, make America great, again, is the definition of going back in time.

And so there is this conflict between, do we want to move forward or do we want to move backward?

That means that, every time we have an election -- and an election is basically a status competition, right?

There's a winner and a loser.

Rather than it's just being our party that wins or loses, now it feels like our racial group and our religious group and our cultural group is also winning or losing.

So that makes the stakes feel a lot higher to us on a psychological level.

We don't have a place to go together, right?

That's much more of a tug-of-war, rather than a negotiation.

JUDY WOODRUFF: Back in a storage room inside the museum, among collections of presidential fine china, history that is not yet fully written or understood.

CLAIRE JERRY: We're always looking for what sort of says the moment.

And these two slogans certainly say the moment of January 6.

JUDY WOODRUFF: Signs collected after the insurrection of January 6, when supporters of President Trump attempted to stop the transfer of power.

Mason's most recent book predicts that our divides today over our identities and competing visions for the country's future will likely lead to more political violence, but that it's ultimately up to our leaders.

LILLIANA MASON: People listen to leaders.

We have run some experiments where we have had people read messages from Joe Biden and Donald Trump, for example, a message that tells them, violence is never OK, we should never engage in violence.

When people read that message, they become less approving of violence.

Our leaders are able to guide their followers toward violence or away from violence.

Whether or not they encourage their supporters to engage in violence is actually up to them.

And our future is going to depend on that outcome.

MICHELLE VITALI: The divisions in the country are definitely causing me a lot of anxiety and lost sleep, but I also -- I'm a hopeful person, and I'm a solution-oriented person, and I'm a person who tries to take action where I can be done.

SUDHANSHU MISRA: A lot can be done organizing at the grassroot level, and we need a leader, someone like Martin Luther King or Gandhi.

KARA ELLIS: I do think there is probably a solution or better days ahead.

I just can't visualize it yet.

And I'm not sure I have the road map or know anybody who has the road map for how to get there.

FABIAN GONZALEZ: Can we get better in time?

God, I hope so.

We better.

JUDY WOODRUFF: For the "PBS NewsHour," I'm Judy Woodruff in Washington.