America is a divided nation, we are told. But what divides us?

In “Asymmetric Politics,” two political scientists — Matt Grossman (Michigan State University) and David Hopkins (Boston College) — compile an enormous amount of data to show that the two political parties aren’t divided by much of anything. Instead, they talk past each other. Republicans talk about issues and Democrats talk about the ways groups perceive themselves.

To understand Democrats, remember that our ancestors lived in tribes that fought with other tribes over food, territory and resources. Loyalty to a group and identification with the group were essential to success. Today, those characteristics are in our genes. That’s why the strongest political force in the world has traditionally been nationalism. Identity politics is the new nationalism.

Think about the Women’s Marches, the protesters at Donald Trump rallies, and the angry mobs currently besieging the Republicans at town hall meetings. Think about all the posters, signs, and messages plastered on tee shirts, hats, and banners. Did you ever see a sign saying “I’m for Free Trade”? How about “Viva NAFTA”? Or “Hooray for TTP”? How about “Open the Borders” or “Let Everybody In”? Did you see any signs addressing any other issue that Donald Trump campaigned on? Veterans’ health? Corporate taxes? Crime on the streets? No?

Welcome to the world of identity politics, which I am going to describe by the acronym GRAVE: Grievance, Retaliation, Action, Validation and Empathy. If you are a Republican and you are used to thinking that politics is about real issues, all of this may be quite new to you.

Grievance defines identity. At the time of the Women’s Marches, I received an email message from a friend, explaining why she was passionately participating. “When I was young I was raped by my father, by my uncle and by a priest,” she texted me. “Trump is a pig,” she added.

I wrote back, “So why aren’t you protesting Hillary and Bill. After all, Bill Clinton is the most infamous abuser of women in the history of American politics and Hillary Clinton is the most infamous enabler of such abuse.”

I never got a response. But the answer is obvious. Time and time again, Hillary told women they were victims. Donald Trump didn’t.

Retaliation feeds identity. The opposite of identity politics is a melting pot, where differences tend to become unimportant. You might think that as discrimination against women, gays, blacks and other minorities evaporates, there would be fewer grievances. And there are. But identity politics needs conflict — unending conflict — if for no other reason than to remind people that there are grievances that need to be redressed.

For example, students at the University of Michigan are demanding a no-whites-allowed space where they can plot “social justice” activism. In their demands, the students explain why a $10 million, new black student center the university has built for them is not enough: it’s open to white students as well.

At New Trier High School, located in a wealthy suburb on Chicago’s North Shore, students are preparing for an all-school seminar on “Understanding Today’s Struggle for Racial Civil Rights.” As an editorial by Peter Berkowitz in The Wall Street Journal explained: “Instead of teaching, the school’s aim seems to be hammering home to students that racism plagues America and will persist until white people admit their unjust privilege, renounce their unearned power, and make amends for the entrenched oppression from which they continue to profit handsomely.”

When parents requested balance, say, by including Hoover Institution scholar Shelby Steele or Wall Street Journal writer Jason Riley, their efforts were met with stonewalling and vitriol.

Action solidifies identity. Successful retaliation means victory. Victories produce victims. And redress of grievances is most satisfying when the victims are personified.

Have you ever wondered why there are so many cases of hapless individuals caught up in the merciless machinery of political correctness? The lecturer who tells students they have the right to choose their own Halloween costume. The photographer who declines to take photographs at a gay wedding. The baker who doesn’t want to put two figures of the same gender on a wedding cake.

For identity politics, these unfortunate individuals come to personify all the many instances of hurt by others through the years. Because of unlucky circumstance, these innocent individuals are forced to redeem the injustices committed by those who went before them.

Robert Ingersoll was a longtime friend and customer of Barronelle Stutzman, owner of Arlene’s Flowers in Richland, Wash. On their last meeting, they even hugged each other. But when Ingersoll asked Stutzman to provide flowers for his gay wedding ceremony, she declined, telling Ingersoll it would violate her Christian faith. Ingersoll sued.

Stutzman’s lawyers argued that since other florists were willing to serve the couple, there was no real harm done. But the Washington State Supreme Court wrote, “[T]his case is no more about access to flowers than civil rights cases in the 1960s were about access to sandwiches.”

This reasoning is quite wrong. If blacks had widespread access to lunch counters in the 1960s, there never would have been a civil rights law. No one cares about the errant beliefs or prejudices of a merchant or two.

Well, almost no one. Identity politics needs conflict. It needs to vanquish those it sees as representing oppression — no matter how small or inconsequential. Trivial victories produce large emotional payoffs.

Validation gives identity self-respect. Milo Yiannopoulos is a comic whose humor is crude and lewd. I suspect most readers would find him disgusting and vile. After revelations of an interview in which he appears to condone pedophilia, CPAC disinvited him to its annual conference, Breitbart fired him, and Simon & Schuster cancelled his book deal.

Here’s why he is important. Even though he is gay, Milo makes jokes about gays … and women … and other minorities. In fact, almost all his humor is “politically incorrect.” Berkeley is the home of the free speech movement. As far as I know, you can still say almost anything on the Berkeley campus — unless it offends someone’s identity, that is. When students learned that Milo was scheduled to speak there, they rioted, caused $100,000 worth of damage, and prevented the appearance.

Identity politics takes itself very seriously. Because it’s so off the wall, it provides a rich smorgasbord of temptations for almost any comic. Laughing at the politics of identity, however, is an unforgivable sin.

Empathy empowers identity. You’ve heard the phrase, “The enemy of my enemy is my friend.” In identity politics, this works like a charm. Virtually every speaker at the Democratic convention last summer clicked off the list of groups they were for: women, blacks, LGBT, etc. What seems to unite them all is the perception of a common but unidentified oppressor: the heterosexual, white male. Shared suffering creates a shared need to cooperate, even among groups that (as Yiannopoulos would point out) don’t actually like each other very much.

Identity backlash. Just about every other day, some writer in The New York Times claims that Donald Trump is sexist, misogynistic, homophobic, xenophobic, racist, etc. (See here, for example.) As I have said before, none of this is true. What is true is that Trump rejects political correctness. He even makes fun of it (an unforgivable sin, remember) and that undoubtedly doubles and triples the amount of hostility on the Left.

Also, Republicans are not immune from identity politics. If the truth were known, they may have benefited from it more than the Democrats in the last election. Trump got a majority of white women votes and a very large majority of white men. Was that because Trump recruited these voters based on white identity? Or because the identity politics practiced by the Democrats drove them to Trump?

Why is any of this important? David Henderson, channeling Robin Hanson, thinks “politics is not usually about policy.” He made this observation in the context of discussing Milo Yiannopoulos and his detractors. Certainly, none of what I am describing here is about rational policies that economists care about and study. But what I am describing definitely affects what happens to you and me at work, at school, in our social circles and even in Washington, DC.