Overestimating how similar other people’s attitudes and beliefs are to one’s own.

Explanation

Assumed similarity bias occurs when individuals overestimate the degree to which other people share their own beliefs, values, attitudes, personality traits, and behavioral tendencies, using the self as the default reference point for understanding others rather than gathering independent evidence. This cognitive mechanism stems from the brain’s efficient but error-prone tendency to anchor judgments in readily available personal experience, a process rooted in the need for social predictability and cognitive ease. Neuroscientifically, it draws on regions involved in self-referential processing, such as the medial prefrontal cortex, which activates both when reflecting on one’s own mental states and when inferring those of others, often blurring the boundary between self and other in the absence of strong countervailing data. The bias operates through egocentric heuristics—mental shortcuts that project one’s internal world outward—combined with limited perspective-taking, leading perceivers to underestimate genuine diversity in human responses. Over time, repeated exposure to similar others in social networks reinforces the pattern, creating echo chambers that feel like objective consensus.

Examples

• U.S. Planning for Post-Invasion Iraq (2003): Senior American officials, including Deputy Secretary of Defense Paul Wolfowitz and Under Secretary Douglas Feith, assumed that Iraqis would respond to regime change much as citizens of liberated European nations had after World War II, embracing democracy, free markets, and gratitude toward liberators. Wolfowitz famously told Congress in February 2003 that “we are dealing with a country that is fundamentally different from the one we dealt with in the past” yet projected that “the Iraqi people will greet us as liberators,” with estimates of only 30,000–50,000 troops needed for stabilization. A declassified February 2003 briefing paper from the Office of Special Plans downplayed sectarian differences, stating that “Iraqis share our aspirations for freedom and prosperity.” When coalition forces toppled Saddam Hussein in April, widespread looting erupted, sectarian violence surged, and an insurgency killed over 4,000 U.S. troops by 2011 while displacing millions. The over-reliance on assumed similarity in preventive nation-building models left intelligence and reconstruction efforts under-resourced for detecting deep tribal and religious cleavages or mounting rapid adaptive responses, turning initial military success into prolonged instability.
• Amazon’s Secret AI Recruiting Tool (2014–2018, Seattle): Amazon’s machine-learning engineers, predominantly mirroring the company’s male, technical, high-achieving culture, trained an AI system on historical resumes assuming that successful candidates would exhibit traits and educational paths similar to their own. Internal documents reviewed by Reuters in 2018 revealed the tool downgraded resumes containing the word “women’s” (as in women’s chess club) and penalized graduates of all-women colleges, reflecting the developers’ unexamined projection of their own career trajectories as the universal standard. By 2015 the algorithm had systematically filtered out female applicants at scale. Amazon scrapped the project in 2018 after internal audits exposed the bias. This assumed-similarity approach to preventive hiring efficiency starved detection of gender-diverse talent signals and response mechanisms like diverse validation datasets, resulting in reputational damage, legal risks, and a narrower talent pipeline that hindered innovation in a competitive industry.
• Cardiovascular Research and Clinical Guidelines Assuming Male Physiology as Default (1970s–1990s, United States): Leading cardiologists and National Institutes of Health-funded researchers assumed that women’s cardiovascular systems functioned essentially as smaller versions of men’s, leading to the routine exclusion of women from major trials and the application of male-derived diagnostic criteria and treatments to female patients. The landmark Physicians’ Health Study (1982–1988), which tested daily aspirin for heart disease prevention, enrolled 22,071 men and zero women; its lead investigators concluded the results applied broadly, with one summary stating benefits for “all at-risk individuals.” Similarly, the Multiple Risk Factor Intervention Trial enrolled 13,000 men only. As a result, women experiencing heart attacks often presented with atypical symptoms such as nausea, fatigue, or jaw pain rather than classic chest pressure, yet guidelines trained physicians on male patterns. Women were 50% more likely to be misdiagnosed during heart attacks, contributing to higher mortality rates. The preventive logic of using male bodies as the universal template starved investment in sex-specific detection tools and response protocols; balanced research inclusion could have produced tailored diagnostics and saved thousands of women’s lives decades earlier.
• Lord Macartney’s Embassy to Qing China (1793, Beijing): British envoy Lord George Macartney and his delegation assumed that the Qing imperial court would approach diplomacy, trade negotiations, and protocol in ways similar to European monarchies, expecting mutual recognition of sovereign equality and commercial openness. Macartney carried a letter from King George III requesting expanded trading rights and a permanent British embassy, projecting British norms of reciprocal diplomacy. A key primary record, Macartney’s own journal, records his expectation that the Chinese “would see the advantages of intercourse with us as clearly as any European nation.” Emperor Qianlong’s famous edict rejected all requests, declaring “We have never valued ingenious articles, nor do we have the slightest need of your country’s manufactures.” The mission ended in failure, with the British party forced to perform the kowtow and departing empty-handed. The preventive emphasis on familiar European diplomatic rituals without adequate cultural intelligence or flexible adaptation starved detection of Confucian hierarchy and isolationist priorities; greater investment in accurate cross-cultural understanding might have avoided decades of escalating tensions that contributed to the Opium Wars.
• British Colonial Administration in 19th-Century India (1850s, Uttar Pradesh): British East India Company and later Crown officials assumed Indian princely states and rural populations operated under loyalty and administrative norms similar to Victorian British hierarchies, projecting their own sense of contractual obligation and gradual reform onto local rulers. Governor-General Lord Dalhousie’s 1854 doctrine of lapse explicitly treated adoption practices in states like Jhansi as incompatible with “civilized” succession, with one dispatch stating, “The people will recognize the benefits of direct British rule as they would in England.” Widespread resentment culminated in the 1857 Indian Rebellion, where sepoys and civilians joined uprisings, resulting in thousands of deaths on both sides and the end of Company rule. Preventive emphasis on anglicization and revenue extraction without robust local intelligence networks or adaptive governance left the administration blind to underlying grievances; greater investment in cultural detection and responsive dialogue might have mitigated the scale of revolt.

Conclusion

Assumed similarity bias carries profound implications for individuals navigating personal relationships, societies crafting policy, academic fields pursuing truth, and future institutions confronting complexity in an interconnected world. As psychologist Lee Ross observed in his foundational work on social perception, humans routinely “see their own behavioral choices and judgments as relatively common and appropriate,” a pattern that inflates overconfidence while blinding actors to divergent realities. Neurobiologically, the bias arises from overlapping neural circuits for self- and other-representation, which prioritize fluency over accuracy when cognitive resources are stretched. Mitigation demands deliberate practices: structured devil’s advocacy, mandatory diversity in decision teams, pre-mortems that force consideration of dissimilar perspectives, and regular calibration against external data. By cultivating epistemic humility and active perspective-taking, decision-makers can transform potential blind spots into sources of resilience. In the end, the clearest vision emerges not from assuming the world mirrors ourselves, but from daring to map its true contours.

Quick Reference

→ Synonyms: false consensus effect; social projection; assumed similarity effect; self-anchoring bias
→ Antonyms: perspective-taking accuracy; realistic divergence recognition; other-centered calibration
→ Related Biases: false uniqueness effect, mirror imaging, confirmation bias, in-group favoritism

Citations & Further Reading

• Choi, I., & Cha, O. (2019). Cross-cultural examination of the false consensus effect. Frontiers in Psychology.
• Human, L. J., & Biesanz, J. C. (2011). Through the looking glass clearly: Accuracy and assumed similarity in well-adjusted individuals’ first impressions. Journal of Personality and Social Psychology, 100(2), 349–364.
• Kenny, D. A. (2021). Interpersonal perception: Assumed similarity. David A. Kenny’s website.
• Renshon, J. (2009). Mirroring risk: The Cuban missile estimation. Intelligence and National Security, 24(3), 315–338.
• Ross, L., Greene, D., & House, P. (1977). The “false consensus effect”: An egocentric bias in social perception and attribution processes. Journal of Experimental Social Psychology, 13(3), 279–301.
• Srivastava, S., Guglielmo, S., & Beer, J. S. (2010). Perceiving others’ personalities: Examining the dimensionality, assumed similarity to the self, and stability of perceiver effects. Journal of Personality and Social Psychology, 98(3), 520–534.