By Jason Hreha

Glossary

Partisan Bias

Reviewed by Jason Hreha

Published 3 min read

A study sounds rigorous when its result helps your party. Change the party label and the same method suddenly looks suspicious. The sample is too small. The researchers must have had an agenda. The conclusion reaches further than the data.

Perhaps those criticisms are correct. The revealing question is whether you would have found them with the labels reversed.

What is partisan bias?

Partisan bias is the tendency for political loyalties to shape how people seek, judge, remember, or explain information. A claim that supports one’s political side may receive an easier hearing. The same claim associated with an opposing side may face closer scrutiny or immediate rejection.

This does not mean that every political belief is irrational. Parties can differ in the quality of their evidence, the effects of their policies, and their commitment to facts. Partisan bias concerns the effect of allegiance on judgment, not the mere existence of political disagreement.

What does partisan bias look like?

It can enter at several points. People may choose congenial news, accept a favorable argument with fewer questions, remember an opponent’s failure more readily, or explain the same conduct differently depending on who did it.

Consider a politician who changes position after new evidence appears. Supporters may call the change pragmatic. Opponents may call it unprincipled. Reverse the politician’s party and the explanations may reverse too. The behavior did not change. The meaning assigned to it did.

A 2025 review of partisan bias in political judgment describes how people can seek, believe, and remember information that supports their political beliefs and affinities. The authors argue that cognitive and motivational factors both contribute. In other words, partisan bias is not reducible to one conscious decision to defend the team.

Are liberals or conservatives more biased?

This question is often answered with a slogan. The research requires a narrower answer.

Peter Ditto and colleagues conducted a meta-analysis of 51 experiments involving more than 18,000 participants. The studies tested a specific form of partisan bias: whether people evaluated otherwise identical information more favorably when it supported their political side. Across those studies, liberals and conservatives did not differ in their mean level of that bias.

That finding does not show that the political left and right are equally biased in every respect. It does not compare every factual belief, policy judgment, media environment, or political system. It says something more useful and less dramatic: in this controlled class of comparison, both groups showed partisan favoritism at similar average levels.

“Both sides” is a slogan. A controlled comparison is evidence about a particular judgment.

Partisan bias and related concepts

Partisan bias overlaps with several better-known ideas, but none is a perfect substitute.

Myside bias is the broader tendency to evaluate evidence in favor of one’s existing position. The position need not be political. Confirmation bias concerns the search for or handling of belief-consistent information. Ingroup bias concerns favorable treatment or judgment of people in one’s group. Partisan bias can involve all three: a prior belief, a political group, and a preference for evidence that helps that group.

It also overlaps with motivated reasoning. But a partisan judgment does not have to be a deliberate defense of identity. Attention, memory, and familiar standards of evidence can produce favoritism without a conscious plan.

Reverse the party labels

Write down why the study, source, or decision passes your standard. Now reverse the party labels without changing the evidence. Does the standard move? Would you accept the method if the result helped the other side? Would you explain the conduct in the same terms?

Then state the rule before looking at the affiliation: What evidence would count? How strong must it be? What result would change the conclusion? A standard chosen in advance has less room to bend around the answer you wanted.

The point is not to become neutral about politics. It is to keep the rule for judging evidence from changing with the team.

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