Trump's impeachment shows the jarring effects of politics on the brain. Here's how to tell if your thinking is overly biased, according to the latest science.

• Trump's impeachment is historic.
• If you already have a political identity, it's unlikely to change your views.
• That's because of how the human brain works.
• We filter the world through our identities.
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Trump's impeachment is historic.

It's only the third time in US history that it's happened.

And if you already identify as a Republican or Democrat (or progressive or conservative or communist or libertarian or liberal or alt-right or dirtbag left), it's not going to nudge your opinion of him one inch.

Because people don't make decisions about things due to some flat evaluation of objective facts, but due to the groups that we belong to. Unavoidably, we perceive the world relative to our personal identities.

Humans are social animals

If you don't belong to a group, you're dead.

Or dying much faster: Massively large scale public health data, some of the widest reaching social science conducted in this country in this century, shows that when people are lonely, they are more likely to die than those who are socially integrated.

Seriously.

In medieval society, being exiled from your town was a fate worse than death. We always have, and probably always will, rely on others in order to survive, maybe even enjoy life, and (crucially) derive some sense of status among our peers.

We are motivated to think in ways that reinforce our status

Psychologists call it motivated reasoning, cognition, or perception. It's all the same bundle of concepts that point to how we tend to see the world filtered through our group identities.

In an early example, way back in the 1950s, Dartmouth and Princeton researchers asked students of their respective universities to watch tape of a particularly brutal football game between the two schools. And guess what: The students reported seeing more penalties committed by the opposite team than their own.

More pressingly and more presently, a research team lead by Dan M. Kahan of Yale Law asked study participants to work out math problems related to hot-button policy issues. The result: People were worse at doing math when it disagreed with their political ideologies, like if a liberal were asked to carry out calculations finding that gun control didn't actually stop gun violence.

That brings us back to the status bit. As fellow data nerd Ezra Klein has pointed out, thinking doesn't happen as some abstracted process. You're being pulled by the many social obligations that pervade our lives. And if you are a part of a more conservative or more liberal community, then you are going to be motivated to profess opinions that uphold your standing in that group. The Princeton students are loyal to their school, so they attend to the game in a way that reflects those values. Nobody wants to be kicked out of the group. Exile is death.

Per Klein, it's not that if people only had more information then they'd be able to make better decisions. Instead, we are all perceiving the world in a way that fits with our values and our feelings about who we are. Before we even get the chance to deliberately reason something out, our identities have shaped our perceptions.

The bias, dear reader, is not simply in our social feeds. But in our identities.

Interpreting Trump, maintaining our selves

After Trump's inauguration, the Washington Post surveyed more than 1,3000 US adults and asked them to indicate whether his ceremony had more attendees than Obama's swearing in. The result: Trump voters were much more likely to say that their candidate had the bigger crowd.

A Gallup poll released Wednesday, the day of impeachment, found that Trump had an 89% approval rating among identified Republicans and a 45% approval rating overall. Psychology tells us that people are going to pick the narrative of what's going on depending on their party identity, like that it was Ukraine that meddled in the 2016 election. But it's not just the right wing that acts this way: Democrats suddenly became much more confident about the economy after Obama was elected, just as Republicans did with Trump. Political polarization is the psychology of identity working at scale.

The mechanics of all this were revealed in brain-image detail in a 2016 paper led by University of California Jonas T. Kaplan. His research team presented arguments to participants that contradicted their liberal beliefs. In accompanying brain scans, they found that an en-vogue set of areas in the brain became active when doing so - something called the default mode network. And what does the default mode network do? According to the best science we have, moment by moment, thought by thought, it's continually creating and maintaining our sense of self. In a nonscientific sense, the science says, politics are part of who we are.