Within Myside Bias
Why Mixed Evidence Can Make Beliefs Stronger
Mixed evidence can make people feel more certain when they judge supportive and hostile findings by different standards.
On this page
- The Lord, Ross and Lepper capital punishment study
- How biased assimilation changes evidence weights
- When mixed results should lower confidence instead
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Introduction
People often assume that exposure to balanced evidence should naturally produce more balanced beliefs. Yet research on myside bias suggests that the opposite can happen. When people encounter a mixture of studies supporting and challenging a strongly held view, they may become more convinced that they were right all along. The reason is not simply that they ignore opposing evidence. Instead, they often evaluate supportive and contradictory findings by different standards, treating favourable evidence as credible while searching for flaws in unfavourable evidence. This process, known as biased assimilation, helps explain why debates over moral, political and social issues can remain polarised even when participants have access to much the same information. [Frank Baumgartner]fbaum.unc.edujpsp 1979 Lord Ross LepperFrank BaumgartnerBiased Assimilation and Attitude Polarization: The Effects…by CG Lord · 1979 · Cited by 7347 — To test these assumpti…
The Lord, Ross and Lepper capital punishment study
The classic demonstration comes from a 1979 experiment by psychologists Charles Lord, Lee Ross and Mark Lepper. They recruited participants who already held strong views either supporting or opposing capital punishment and presented them with evidence about whether the death penalty deters murder. Rather than showing only one side, the researchers deliberately presented mixed evidence.
Participants first read summaries of two fictional studies:
- one apparently showing that capital punishment reduced murder rates;
- one apparently showing that it did not.
Afterwards they received more detailed descriptions of each study’s methods and results. Importantly, both studies contained strengths and weaknesses, giving participants reasonable material for critical evaluation.
Instead of moving towards the same conclusion, supporters and opponents became more polarised. Each group rated the study supporting its prior beliefs as better designed, more convincing and more informative, while identifying methodological flaws primarily in the opposing study. After reviewing the entire body of evidence, participants often reported greater confidence in their original position than before the experiment began. [Frank Baumgartner]fbaum.unc.edujpsp 1979 Lord Ross LepperFrank BaumgartnerBiased Assimilation and Attitude Polarization: The Effects…by CG Lord · 1979 · Cited by 7347 — To test these assumpti…
The study became influential because it demonstrated that disagreement can persist even when everyone examines the same information. The difference lies less in what evidence people receive than in how they judge its quality.
How biased assimilation changes evidence weights
The experiment illustrates a simple but powerful psychological mechanism: people rarely assign equal weight to all evidence.
Instead, they unconsciously apply asymmetric standards.
- Supporting findings receive the benefit of the doubt. Minor methodological problems are overlooked or judged unimportant.
- Contradictory findings receive intensive scrutiny. Small weaknesses become reasons to dismiss the entire study.
- Ambiguous findings are interpreted in the direction already favoured. The same uncertainty points towards opposite conclusions depending on the reader’s prior belief.
- Memory also becomes selective. People tend to remember the strongest arguments supporting their own side while recalling mainly the weaknesses of opposing evidence. [Wiley Online Library]compass.onlinelibrary.wiley.comj.1751 9004.2009.00203.xWiley Online LibraryBiased Assimilation: Effects of Assumptions and Expectations…23 Sept 2009 — Biased assimilation occurs when percep…
This process differs from simply seeking confirming information. Even when people genuinely read both sides, they may still emerge more certain because they have effectively given different evidential weights to the same material.
Researchers describe this as biased assimilation because new information is assimilated into existing beliefs rather than evaluated from a neutral starting point. The bias usually operates without deliberate dishonesty; people often believe they are being objective while applying stricter standards only to evidence they dislike. [Wiley Online Library]compass.onlinelibrary.wiley.comj.1751 9004.2009.00203.xWiley Online LibraryBiased Assimilation: Effects of Assumptions and Expectations…23 Sept 2009 — Biased assimilation occurs when percep…
Why mixed evidence can increase confidence
Mixed evidence often creates an illusion of careful evaluation.
Someone may think:
“I looked at both sides and still reached the same conclusion.”
That conclusion feels more trustworthy because it appears to have survived scrutiny. The problem is that the scrutiny was uneven.
If supportive evidence receives a credibility score of “8 out of 10” while contradictory evidence is mentally downgraded to “2 out of 10”, the overall impression becomes overwhelmingly one-sided even though the objective evidence base was mixed.
This is particularly likely when beliefs are connected to:
- moral values;
- political identity;
- group membership;
- public commitments;
- professional reputation.
In these situations, accepting contrary evidence carries social or psychological costs, increasing motivation to find reasons why inconvenient studies are flawed rather than informative. [Wiley Online Library]compass.onlinelibrary.wiley.comj.1751 9004.2009.00203.xWiley Online LibraryBiased Assimilation: Effects of Assumptions and Expectations…23 Sept 2009 — Biased assimilation occurs when percep…
The mechanism also helps explain why two intelligent people can read the same scientific literature and become even more convinced that the evidence favours opposite conclusions.
When mixed results should lower confidence instead
From an analytical perspective, genuinely mixed evidence usually argues for greater uncertainty, not greater certainty.
When high-quality studies point in different directions, a more evidence-sensitive response is often to conclude that:
- the effect may be smaller than expected;
- important conditions or contexts remain unidentified;
- existing studies have methodological limitations;
- additional evidence is needed before drawing firm conclusions.
Scientific reasoning generally rewards proportional confidence. Confidence should rise when independent evidence consistently converges and fall when comparable evidence conflicts.
The lesson from biased assimilation is therefore not that balanced evidence is ineffective, but that readers need consistent evaluation standards. Questions such as Would I criticise this design if it supported my own view? or Would I find this evidence persuasive if the conclusions were reversed? help counter the tendency to apply asymmetric standards.
What later research changed—and what it did not
The Lord, Ross and Lepper study remains one of the most influential demonstrations of biased assimilation, but later research has refined its interpretation.
In particular, researchers have questioned the broader idea that factual corrections routinely produce a backfire effect, in which people become even more committed to false factual beliefs after correction. Large-scale experiments involving more than 10,000 participants across dozens of contested issues found little evidence that factual corrections reliably strengthen misinformation. Reviews likewise conclude that strong factual backfire effects appear to be uncommon. [SSRN+2PNAS]papers.ssrn.comThe Elusive Backfire Effect: Mass Attitudes' Steadfast…by T Wood · 2017 · Cited by 1175 — Across all experiments, we found no corr…
This refinement does not undermine the central lesson about mixed evidence. The stronger conclusion is narrower but better supported:
- people often do update beliefs in response to credible evidence;
- however, they frequently evaluate congenial and uncongenial evidence differently;
- unequal evaluation can preserve or even strengthen existing attitudes despite exposure to the same information. [PNAS+2PMC]pnas.orgWhy the backfire effect does not explain the durability of…by B Nyhan · 2021 · Cited by 330 — Previous research indicated that cor…
For improving analytical thinking, this distinction matters. The greatest obstacle is often not refusing to look at opposing evidence, but unconsciously judging identical standards differently depending on whether the evidence confirms or challenges what we already believe.
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Further Reading
Books and field guides related to Why Mixed Evidence Can Make Beliefs Stronger. Use these as the next step if you want deeper reading beyond the article.
The Scout Mindset
Addresses biased assimilation and how to update beliefs when evidence is mixed.
Mistakes Were Made (but Not by Me)
Shows how self-justification strengthens beliefs despite conflicting information.
Thinking, Fast and Slow
Explains why people systematically misjudge evidence and remain overconfident.
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Endnotes
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Source: compass.onlinelibrary.wiley.com
Title: j.1751 9004.2009.00203.x
Link: https://compass.onlinelibrary.wiley.com/doi/10.1111/j.1751-9004.2009.00203.xSource snippet
Wiley Online LibraryBiased Assimilation: Effects of Assumptions and Expectations...23 Sept 2009 — Biased assimilation occurs when percep...
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Source: papers.ssrn.com
Link: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2819073Source snippet
The Elusive Backfire Effect: Mass Attitudes' Steadfast...by T Wood · 2017 · Cited by 1175 — Across all experiments, we found no corr...
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Source: pnas.org
Link: https://www.pnas.org/doi/10.1073/pnas.1912440117Source snippet
Why the backfire effect does not explain the durability of...by B Nyhan · 2021 · Cited by 330 — Previous research indicated that cor...
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Source: pmc.ncbi.nlm.nih.gov
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC9283209/Source snippet
backfire effect after correcting misinformation is strongly...by B Swire-Thompson · 2022 · Cited by 114 — The backfire effect is when be...
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Source: fbaum.unc.edu
Title: jpsp 1979 Lord Ross Lepper
Link: https://fbaum.unc.edu/teaching/articles/jpsp-1979-Lord-Ross-Lepper.pdfSource snippet
Frank BaumgartnerBiased Assimilation and Attitude Polarization: The Effects...by CG Lord · 1979 · Cited by 7347 — To test these assumpti...
Additional References
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Source: karunapsychologicalservices.com
Link: https://karunapsychologicalservices.com/confirmation-bias-relief/Source snippet
Confirmation Bias: Relieve Suffering Through AwarenessThe study demonstrated that the subjects in their experiment had a “biased assimila...
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Source: osf.io
Link: https://osf.io/download/ba2kcSource snippet
counter-arguments consistent with their pre-existing views to contradict the new information or correction (Nyhan &...
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Source: studocu.com
Link: https://www.studocu.com/en-gb/document/university-college-london/introduction-to-psychology/jpsp-1979-lord-ross-lepper/21807450Source snippet
Prior Theories on Subsequently Considered Evidence. Charles G. Lord, Lee Ross, and Mark R. Lepper.Read more...
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Source: gwern.net
Link: https://gwern.net/doc/psychology/cognitive-bias/2018-wood.pdfSource snippet
despite testing precisely the kinds of polarized issues where backfire should be...
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Source: semanticscholar.org
Link: https://www.semanticscholar.org/paper/Why-the-backfire-effect-does-not-explain-the-of-Nyhan/ff9b7e09122bb1e9e2b340239405f5ace64de17dSource snippet
of corrective information like fact checks often do not last or accumulate...
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Source: researchgate.net
Link: https://www.researchgate.net/publication/232555483_Biased_assimilation_and_attitude_polarization_The_effects_of_prior_theories_on_subsequently_considered_evidenceSource snippet
xamine relevant empirical evidence in a biased manner.Read more...
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Source: researchgate.net
Link: https://www.researchgate.net/publication/350778736_Why_the_backfire_effect_does_not_explain_the_durability_of_political_misperceptionsSource snippet
There is some evidence of the durable correction effects of...
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Source: academia.edu
Link: https://www.academia.edu/14157235/Biased_assimilation_and_attitude_polarization_The_effects_of_prior_theories_on_subsequently_considered_evidenceSource snippet
xamine relevant empirical evidence in a biased manner...
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Source: youtube.com
Title: Yuan Chang Leong, Ph D: An integrative view of motivated cognition
Link: https://www.youtube.com/watch?v=Kz-_lqCa8ocSource snippet
Cognitive biases in risk management - Confirmation bias - Alex Sidorenko...
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Source: youtube.com
Title: How to Convince Someone That You’re Right (Even If You’re Not)
Link: https://www.youtube.com/watch?v=Z5629AI_30YSource snippet
Yuan Chang Leong, PhD: An integrative view of motivated cognition...
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