Editor’s Note: Martin Bisgaard is an assistant professor in the Department of Political Science at Aarhus University in Denmark. His research deals with how partisanship influences citizens’ political thinking, and his recent work has appeared in American Journal of Political Science. The views expressed in this commentary are his own. View more opinion on CNN.

CNN  — 

On September 24, House Speaker Nancy Pelosi announced an impeachment inquiry into President Donald Trump, following his controversial call with Ukraine President Volodymyr Zelensky.

In the July call, Trump asked Zelensky to do him a “favor” and launch an investigation into 2020 rival Joe Biden and his son, Hunter. Was Trump committing an impeachable offense, or was he doing what world leaders do on occasion – asking an ally for assistance into potential corruption?

Martin Bisgaard

The answer to that question depends less on the facts in front of you – and more on what you already think about Trump. For those who don’t approve of Trump, the log of the call supplied by the White House, the subsequently released text messages between US diplomats and the public depositions, further prove what they have long thought about the President: that Trump does not respect the office he holds and that he is using the presidency to further his personal interests.

For those who support the President, the call, text messages and depositions prove almost the exact opposite: that Trump did nothing wrong in asking a foreign ally to look into a potential case of corruption. Indeed, how can the President be impeached for putting pressure on a foreign leader, when the Ukraine President himself denied that this was what happened?

This example illustrates an important point about political reasoning in an era of polarized politics: Facts appear to have little influence on voters’ opinions. Republicans and Democrats have the same evidence at their disposal – the log of the call, the text messages and the depositions – and still they reach opposite conclusions about whether the President should be impeached.

At first glance, it might not seem problematic that people with opposing views arrive at different conclusions from the same set of facts. Yet, the issue is that people do not always examine the facts first and then arrive at a conclusion. They do the exact opposite – deciding on a conclusion and then twisting the facts to support it.

If we think Trump is the biggest threat to American democracy, we can find wrongdoing in a turn of phrase in a phone call. If we believe that Trump is a leader in democracy and morality, we can easily explain why there is nothing wrong in asking a foreign leader to investigate potential wrongdoing.

So, why does this happen? Political psychological research suggests that reasoning is not always geared toward finding the “right” conclusion – whatever the conclusion might be – from a given set of facts but rather used to confirm a predefined one.

We want to confirm what we already believe, or what psychologists refer to as confirmation bias, and we do this by collecting and interpreting evidence in ways that support our prior beliefs.

In terms of politics, this means that partisans want to confirm their existing beliefs on political issues and favor politicians who represent those beliefs. For example, once Republicans have decided that Trump is doing a good job as president, they will find reasons for why new evidence, like a log of a phone call, is consistent with their view of his leadership.

This tendency is not just limited to the Trump era or one particular side of the political aisle. Three years ago, as former President Barack Obama was ending his term, I conducted a large-scale randomized experiment among self-identified Democratic and Republican voters regarding their interpretation of new numbers on economic growth.

The goal of the experiment was to expose participants to information that either showed economic growth was strong or sluggish, thus reflecting positively or negatively on the President.

The results showed that while Republican and Democratic voters were willing to accept the facts at hand and adjust their beliefs about the state of the economy in the direction of the new economic evidence, they were still remarkably apt in explaining away these facts to match their preexisting partisan beliefs.

When confronted with positive growth, Democrats were eager to attribute responsibility to Obama’s economic policies, while Republicans were prone to alleviate him of responsibility, pointing instead to the success of US companies and trends in global markets.

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    When the exact same indicator pointed in a negative direction, voters reasoning patterns flipped: Democrats suddenly mustered up reasons for why Obama was not at fault, while Republicans blamed Obama’s economic policies.

    It is not that these arguments are wrong in and of themselves. The point is that we use such arguments opportunistically – to make new facts fit with a conclusion that we have already decided on, in this case that Obama was good or bad for the economy.

    On rare occasions, of course, irrefutable facts do change the course of history. Just think about how the Watergate scandal eventually brought down former President Richard Nixon. The Gallup polls from the time also tell a remarkable story of how public support for impeaching Nixon grew as the revelations mounted, suggesting that facts can make a difference.

    But what is also remarkable about the Nixon period is that even after all evidence was on the table – including the tapes that directly proved how Nixon had attempted to block investigations – only 58% of adult Americans supported impeachment and a majority of Republicans still supported him. Sometimes, facts can make a difference – but not to everyone.

    Think about how you would reason, if the pressure for impeaching Trump mounted further in the weeks to come.

    What if the taped call with the Ukrainian President was released in full, and it became apparent that Trump did – or did not – ask Zelensky for an explicit quid pro quo? Would either fact change your opinion on Trump? Probably not. Chances are you concluded Trump should or should not be impeached long ago – and you will likely find creative ways of making the facts fit with this conclusion.