This willingness to adopt a policy position not out of conviction but out of a desire to conform to partisan priorities reflects an urge, they write, “to bolster and protect valued political identities by expressing and rationalizing the viewpoints cued to be consistent with these identities.”

In an email, Lelkes wrote that he and his co-authors felt it important “to be careful about causal reasoning.” They concluded their article with this commentary:

Ironically, reflective citizens, who are sometimes seen as ideal citizens, might be the subset of strong partisan identifiers most likely to fall in line with the party. Since higher levels of cognitive resources and partisan social identity are associated with higher levels of political activism, the effect may be self-reinforcing, wherein political elites polarize the strongly identified and cognitively reflective, who then elect more polarized elites. The democratic dilemma may not be whether low information citizens can learn what they need to know, but whether high information citizens can set aside their partisan predispositions.

These two papers raise a set of basic questions about group identity: Why are partisan divisions sharpening now and are these conflicts likely to become more aggravated?

Robert Kurzban, the author of “The Hidden Agenda of the Political Mind,” argues that domestic politics have taken on a moralistic, judgmental cast:

There is a rise in “purity tests” for politicians, such that if they do not adopt the line of the party on core issues — especially those to do with identity politics — they are subject to round condemnation.

Any deviation, Kurzban continued in an email, is likely to be severely punished:

Being just a tiny bit out of step is seen as grievous a sin as being completely out of step. People are seeing others’ positions as categorical rather than continuous.

Steven Pinker, a professor of psychology at Harvard, argues that it is the activation of group identity — of the friend-enemy type, about which I have written before — that is driving hostile polarization:

Certainly there is a tribal flavor to political polarization. Men’s testosterone rises or falls on election night, depending on whether their side wins, just as it does on Super Bowl Sunday.

But, he continued,

the coalitions clustering at the poles are not tribes in the classical anthropological sense. Today’s left- and right-wingers for the most part aren’t inventing myths of shared blood and common ancestry, or binding together in ritual ordeals, or blending in appearance with a common uniform.

Instead, Pinker argued,

I think we’re seeing a somewhat different psychological phenomenon: dynamically sorting ourselves into coalitions defined by moralistic condemnation of designated enemies.

Pinker cited the questions raised in the work of Kurzban and Peter DeScioli, a political scientist at Stony Brook, wondering

why so much of our moralizing does not consist in pondering how to universalize the maxim of our actions or to bring about the greatest good for the greatest number, but rather of condemning, demonizing, or scapegoating a designated sinner.

DeScioli provided some answers to the question in his replies to my queries:

Our coalitional psychology goes far beyond a simple distinction between us and them. We form coalitions nested within coalitions, and our loyalties can suddenly shift. Our minds keep track of other individuals’ allies and enemies in an ornate network of loyalties. Conflicts routinely occur within a group, and generally more often than against outgroups. We use aggression as a tactic both within and between groups, and we do so cautiously, weighing the costs and benefits. Our coalitional psychology is more like a Machiavellian playbook of strategies than a simple reflex of hostility toward an outgroup. Partisan hostility is certainly a form of coalitional aggression rooted in evolved strategies, but so are the prudent strategies of peace and compromise.

Moral condemnation, DeScioli wrote, is a

prime suspect behind partisan hostility. Moral judgment inspires outrage and a desire to punish wrongdoers, forming a potent source of aggression. I have argued that moral judgment is an evolved strategy for choosing sides in conflicts. We seek retribution not only to deter harmful behavior but also to show which side we are on in a dispute.

Moral condemnation and approbation can both be useful to encourage productive cooperation within a group, but they can also be deeply destructive when used to demonize and humiliate outsiders.

John Hibbing, a political scientist at the University of Nebraska, makes the case that polarization has intensified because

the central political issues of the day revolve around in-group versus out-group, the definition of the in-group, and the unity and security of the in-group.

While as recently as 30 years ago, there were a fair number of conservative Democrats and liberal Republicans who held views contrary to the majority of their respective parties, Hibbing wrote,

the problem today is that there are so few cross-cutting cleavages. There is only one cleavage and it is the most evolutionarily primal cleavage of them all.

Hibbing dismissed the argument that people have “become tribal because of changes in the media environment.” Instead, he wrote,

people became tribal because the fundamental substantive issues today are about tribe. We are a group-based species.

Kevin Smith, who is also a political scientist at the University of Nebraska, argues that

humans are incredibly groupish creatures — as a species we’re pretty hard wired to make “us v them” categorizations.

For much of the twentieth century, according to this line of thinking, the first world war, the Great Depression, the second world war and the Cold War filled much of the need for an in-group versus out-group configuration. During this period, Smith said, there were consensual “sources of information” and political parties “that were relatively nonideological” with many members “willing to cross the aisle to cut deals.”

In recent years, “a lot of that has gone,” Smith told me. “Political parties are now deeply ideological,” which makes it easier to

appeal to our “us versus them” instinct. Indeed, it makes it more and more likely that we emotionally attach ourselves to a particularly political group, remain reflexively loyal to it, and automatically denigrate anyone who disagrees with it — we are indeed hard-wired for this sort of thing and political parties have, in effect, organized themselves around, and are now appealing directly to, that part of our psychology which is, and always has been, more emotional than rational.

When, if ever, will things improve? Jonathan Haidt, a psychologist at N.Y.U., is not optimistic. He emailed:

I am expecting that America’s political dysfunction and anger will worsen, and will continue to worsen even after Donald Trump leaves the White House.

Why?

The reasons for my pessimism are that 1) social media gets ever more effective at drowning us in outrage; 2) overall trust in institutions continues to decline, which makes it seem ever more urgent that “our” side take total control; 3) the younger generations have not seen effective political institutions or norms during their lives, and also seem less adept at handling political disagreements; and 4) the norms of campus regarding call-out culture seem to be spreading quickly into business and many other institutions.

Two of the most insightful people on evolved human behavior are Leda Cosmides and John Tooby, professors of psychology and anthropology at the University of California, Santa Barbara.

Cosmides and Tooby explore the complexities and ambiguities of moral reasoning in their seminal essay “Groups in Mind: The Coalitional Roots of War and Morality,” which concludes on an appropriately ambiguous note:

The set of evolved programs that enable and drive warfare and politics strongly overlap with the set of evolved programs that drive human morality. The mapping of these evolved programs and their embedded circuit logic is only in its infancy, and we have only sketched out some of the known or predicted features of our coalitional and moral psychologies. However, progress in this enterprise holds out the possibility of gradually throwing light on some of the darkest areas of human life.

In a lengthy email that I cannot fully do justice to in this column, Tooby wrote that everybody

benefits from participating in groups of alliances and factions on different scales, and people also benefit by fractionating solidarity in such a way that those on the far side of the boundary seem undesirable, worth spurning, contemptible, deplorable.

This, in turn, encourages evolved mental programs that support self-advantageous information warfare: Of all the signals an individual hears, and out of all the spontaneous thoughts that teem in our minds, each individual filters, remembers, and broadcasts what benefits her in building up herself and her allies, and downgrades those who are her rivals.

Tooby added: “I think we cannot begin to appreciate how immense and all-encompassing this process is.”

Tooby pointed to group pressure to adopt down the line agreement on issues when there is, in fact, much room for dissent and disagreement:

There is no necessary reason why someone’s position on abortion should predict their position on global warming should predict their position on welfare should predict their position on school choice should predict their position on illegal immigration. These are all entirely logically independent, yet there is a natural tendency for alliance gravitation to pull people into sets — often binary sets — because issues are more often flags of identity, and it creates in-group dissension to have a multiplicity of views inside the group.

Even more important, he writes,

Honest reasoning about issues is inconsistent with group loyalty. To be a good group member, I should adhere to a position because it is the group’s position, while believing that the facts justify it.

Leda Cosmides expressed some of her ideas about the connection between us-and-them thinking and evil, the topic with which I began this column, in an interview that was originally published in a Chilean newspaper in 2001 and still resonates:

The world has many people with evil motives, who will twist whatever ideas are around them to their own ends. Hitler, for example, was more influenced by folk notions about “blood” (found everywhere) than by any real biological knowledge.

She went on:

Since the Enlightenment, people have been trying to build bridges between disciplines, and when they do, new insights are achieved and humanity benefits. Should the healing arts have been kept separate from biology? If they had, we would not have antibiotics and modern medicine. Should psychology be kept separate from biology? If it is, we will never understand how the mind works. As a result, we will never understand how to make war less likely, how to cure autism, how to help people understand risk, or how to prevent racism, to name just a few problems on which evolutionary psychologists are making progress. Indeed, if we keep psychology separate from biology, people will continue to believe that “race” is a sensible concept; in contrast, human population biology tells us that humanity is not divided into distinct “races.”

This work is far from done.

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