Imagine you're at a job interview, where you're asked to assess two equally qualified candidates for a role. One has a background that mirrors your own, while the other comes from a different community or cultural background. Even though both candidates have the same qualifications and experience, you might unknowingly make assumptions about their abilities based on your own biases—whether they relate to gender, race, or even where they grew up. This isn't necessarily intentional, but these assumptions can influence your decision-making process, often leading to unfair judgments or unequal treatment.

This example highlights how prejudice works, shaping our perceptions and behavior in ways we don’t even realize. Prejudice often starts with stereotypes—assumptions about people based on their identity—whether that’s their race, gender, age, or any other characteristic. These assumptions are influenced by our perceptual set, the way our past experiences and expectations color our interpretations of new information. We may also act on preconceived notions about others, which can lead to unfair judgments or unequal treatment, often without us being fully aware of it. Recognizing these biases is an essential first step in reducing their impact on our interactions and decisions.

Stereotypes: Preconceived ideas about an individual based on what you assume people from the group they belong to are like. Stereotypes are generalized assumptions about people based on certain characteristics such as gender, race, culture, or age. We use stereotypes to categorize information and free up brain processing power, and they can also help us make decisions when we encounter new and unknown information. However, these assumptions can be incorrect and are often unfair.

Perceptual Set: Our psychological tendency to interpret new information based on past experiences and expectations. Our expectations help us to understand and classify new stimuli, enhancing brain processing power, but sometimes leading to incorrect judgments. 

Ageism: Prejudice based on age, particularly believing someone to be either too old or too young to be successful in a position or activity. For example, people may assume that someone over the age of 60 would have poor digital skills, and they may be passed over for training opportunities.¹

Sexism: Prejudice based on gender, typically the belief that women are inferior and will perform worse in a position or activity. For example, people may prefer to hire a male candidate in a leadership role over a female candidate.¹

Racism: Prejudice based on race that contributes to a negative sentiment towards people of a racial or ethnic group. For example, historically, prejudiced beliefs that Black people are more violent than white people have been commonplace.¹ 

Implicit Bias: Unconscious feelings and attitudes towards a particular group that can lead to prejudice.¹

Explicit Bias: Conscious feelings and attitudes towards a particular group that lead to intentional prejudiced actions against them. 

The study of prejudice is rooted in ancient philosophy. Roman philosopher and lawyer Cicero used the Latin word praeiusticium, the origin of the English word prejudice, in his legal arguments to describe how a preceding judgment could influence legal decisions. Cicero conceptualized praeiusticium as the opposite of truth, though he never claimed that it was either a positive or negative prior judgment, but one that could lead to errors in legal decisions.³ 

During the Renaissance between the 14th and 17th centuries, there was a renewed focus on gathering empirical observations and challenging whether any knowledge was really inherited. It was during this time that people started to wonder how personal biases influence perception. Philosophers like Francis Bacon began to question whether we could accept “truths,” or whether these were inferences based on personal lived experiences.⁴ In his most famous work, Novum Organum, Bacon identified four false notions, called “idols,” that shape human understanding. He explained that human understanding “is like a false mirror, which distorts and discolours the nature of things by mingling its own nature with it.”⁵

In 1821, English essayist William Hazlitt wrote “On Prejudice,” marking its evolution from the Latin root, where he suggested that prejudice is caused by “ignorance, malice or perversity,” that causes people to hold strong beliefs even in the face of contrary evidence.⁶ 

However, it wasn’t until the 1950s that psychologists began to take an interest in prejudice. After he was recruited by the Boston Police Department to understand the ethnic tensions driving a rise in major riots, American psychologist Gordon Allport explored the psychological mechanisms behind prejudice in his 1954 book The Nature of Prejudice.. Allport concluded that prejudice is a byproduct of our brain’s limited mental capacity; because there is so much information for us to process, our brain relies on stereotypes and categorization to interpret new information.⁷ Categories allow for quick identification to guide our decisions; however, Allport argued that prejudice irrationally categorizes people based on emotions and personal values rather than logic.⁸ 

In 1979, Polish social psychologist Henri Tajfel and British psychologist John Turner took Allport’s theories a step further to explain why prejudice exists not only in individuals but between groups. They developed the social identity theory, which suggests that group membership influences our personal identity, and so to strengthen our in-group membership, we view other groups as threats, causing prejudice.⁹ 

In the late 20th century, there was a shift from studying explicit prejudice—conscious and overt biases—to implicit prejudice, which operates outside of conscious awareness. In 1998, American psychologist Anthony Greenwald and Indian psychologist Mahzarin Banaji created the Implicit Association Test, a tool that measures unconscious bias based on reaction times associating positive and negative words with images.
Today, there is a lot of focus on how prejudice is embedded into technology and artificial intelligence. Despite hopes that computers would not mimic our tendency towards bias, algorithms can carry forms of prejudice inherited from humans, such as an AI recruiting tool that favored male candidates due to the company’s historical hires.¹⁰

Francis Bacon

Lord Chancellor of England in the 16th and 17th centuries, Bacon was a leading figure in philosophy. Bacon believed that much of what was considered “understanding” was founded on personal beliefs, which he categorized into four false notions. The “idol of the cave” explained how prejudice comes from personal experiences that color the way we interpret the world.⁵ Bacon also laid the foundation for modern scientific methodology, as a way to counter false beliefs, through his Baconian method that sets standards for evidence-based reasoning.¹¹ 

William Hazlitt

A British essayist whose work focused on human experiences, Hazlitt was anti-establishment, liberal, and an advocate of freedom, which influenced his writing.¹² He wrote vehemently against prejudice in his 1821 essay “On Prejudice,” which suggested our biased views against other groups were ignorant and irrational.6 Arguably, Hazlitt’s most famous essay is “The Fight,” which detailed his observations about a boxing match he attended.¹³ 

Gordon Allport

An American psychologist who developed the theory that prejudice was caused by our brain’s tendency to categorize information to conserve energy.8 He coined the term “perceptual set,” which describes how our previous experiences shape our perception of new information. Much of Allport’s research focused on personality, and he believed that prejudice was a way for people to strengthen and protect their sense of self by discriminating against people who are different.¹⁴

Henri Tajfel & John Turner:

Polish and British psychologists, respectively, who developed the social identity theory through a series of studies that showed that categorizing individuals into random groups was enough to create group membership and prejudice against an “other” group.¹⁵ They hypothesised that group membership influences how people react in social situations, as it determines how people relate to others. 

Anthony Greenwald & Mahzarin Banaji

American and Indian psychologists, respectively, who founded Project Implicit, a collaboration of researchers interested in implicit bias.¹⁶ They developed the Implicit Association Test to show that even when people are not consciously prejudiced, they may still harbor unconscious bias. 

Prejudice impacts the way that people are treated in critical areas such as healthcare, the workplace, and education. Prejudice can shape social structure and reinforce systemic inequalities. These biases—whether implicit or explicit—can limit opportunities, impact well-being, and perpetuate disparities that affect marginalized groups.

Healthcare

Unfortunately, prejudice against certain races leads to health disparities between groups. There is unequal treatment and diagnosis for marginalized groups, leading to poorer health outcomes. For the fifteen leading causes of death in the U.S., African Americans have higher death rates than white people.¹⁷ Prejudice has a part to play in this disparity—for example, studies show that Black people are systematically undertreated for pain, as doctors are less likely to “believe” the pain that Black people are in.¹⁸ 

Women also face prejudice in the healthcare system. Until 1993, women were rarely included in clinical trials, which means that we cannot be sure how treatments and drugs will impact women. Females have different physiologies, metabolism, and hormone cycles, which means what leads to positive outcomes for men may not have the same effect on women.¹⁹ Additionally, women are more likely to be dismissed by doctors and not have their concerns taken seriously, as they are assumed to be more emotional and hysterical than men. In fact, when the concept of hypochondria was developed, it was thought to be a solely female disease caused by a “wandering womb.”²⁰ 

Workplace

Prejudice often rears its ugly head in the workplace. Over 90% of U.S. workers have reported facing some kind of discrimination in the workplace based on their race, gender, disability, or age.²¹ Women have long faced prejudice in their careers, demonstrated by the wage gap: in 2024, women earned around 85 cents for each dollar a man earned. This discrimination is not based on performance, but on false assumptions that women are inferior employees, which causes them to be compensated less and passed up for promotions, even though research shows that gender-diverse companies are more successful. McKinsey reports exploring the impact of diverse workplaces showed that the most gender-diverse companies outperform the least gender-diverse companies by 48%.²² 

Although we sometimes perceive artificial intelligence as an objective tool free of prejudice, Amazon’s AI recruiting tool proved otherwise. Amazon developed a tool to review resumes to improve efficiency, but the algorithm was biased toward male candidates. It would favor resumes with male names and masculine language, such as “executed” or “captured,” forcing Amazon to shut down the project.¹⁰

Education

Although more educated people tend to be less prejudiced, there is a lot of prejudice embedded within our education systems. The structure of schooling itself has historically reflected dominant cultural values, often marginalizing or disregarding the knowledge and experiences of minority groups. In many cases, this exclusion has reinforced stereotypes and contributed to systemic barriers that still persist today.²³

For example, throughout history, Indigenous peoples have faced prejudice in the education system. In Canada, residential schools were established to assimilate Indigenous children into Euro-Western culture, believing it was superior and that Indigenous pedagogies were simplistic and of a lower standard.²⁴ Indigenous ways of knowing have been devalued, and these colonial beliefs continue to lead to prejudice in education today. For example, Indigenous students often face greater disciplinary action compared to non-Indigenous students: they are two times more likely to be suspended or expelled.²⁵

While prejudice is often discussed in negative terms, its origins and functions are more complex. Some researchers argue that prejudice can serve cognitive, evolutionary, and social purposes, helping people navigate the world efficiently, avoid potential threats, and strengthen group identity.

Making Decisions with Limited Cognitive Ability

Although prejudice can lead to discrimination and unfair treatment of other groups, some of the mechanisms behind prejudice are necessary for us to make quick decisions based on the information we have available to us. Time, brain processing power, and available information all impact our ability to make decisions, and sometimes we have to settle for bounded rationality: making decisions that are good enough rather than optimal.

Prejudice is often based on stereotypes, preconceived ideas of what a person might be like based on the group they belong to, which derive from a categorization method our brain uses to efficiently make sense of information. While stereotypes can lead to prejudice, they may be necessary in situations where decisions have to be made quickly. For example, if you are a woman walking alone at night and see a large male figure following you, stereotypes and prejudice might trigger your sense of safety, allowing you to make decisions to ensure you make it home unharmed. While social categorization can be a useful tool, when we assign negative characteristics to these groups and act in accordance, it leads to harmful prejudice. 

Evolutionary Survival Mechanism

Some psychologists suggest that prejudice was a form of common sense for our ancestors. In prehistoric times, people who did not belong to your tribe or group could threaten the group’s safety. This was because resources were limited, and outsiders could harm (physically or through disease) or steal from the group. Prejudice might therefore have been a protective mechanism allowing people to quickly identify threats.²⁶ Those individuals who were able to identify and respond to threats were more likely to survive and reproduce, passing on a predisposition for prejudice to future generations.²⁷ 

Unfortunately, this psychological tendency remains even in the modern world, where most of the time, other people do not pose an immediate threat to us, yet we still harbor negative sentiments toward other groups.²⁶

Strengthens Group Identity

While prejudice causes people to negatively perceive and sometimes poorly treat “others,” it also causes in-group favoritism. Historically, for our survival, it was beneficial for us to work with others—cooperation allowed for protection and the sharing of resources.²⁷ Prejudice against others could lead to trust and loyalty from the in-group. Even in more modern times, research has shown that people prefer individuals who express in-group favoritism. Italian psychologists Luigi Castelli and Luciana Carraro conducted a study that showed psychology students preferred in-group members who gave more points to the in-group and the least amount of points to the out-group.²⁸

There are some harmless ways that this in-group bias can contribute to a sense of belonging. For example, imagine that you are attending a soccer match in a foreign country. By wearing a local team jersey, you are signaling to others that you are part of their group, which will make them perceive you in a more positive light. 

The Doll Test

One of the most famous studies exploring the impact of prejudice was conducted by psychologists Kenneth and Mamie Clark in the 1940s. The psychologists were interested to see the effects that racial segregation had on young African-American children.

African-American children between the ages of three and seven participated in a study where they were shown four dolls, each with different skin colors. Children were asked to identify the race of each doll, which doll resembled them, and which doll they preferred. 

Most children, aware that they looked most like the Black dolls, preferred the white doll. For prompts that asked the children to identify a doll with positive characteristics, such as “Give me the doll that is a nice doll,” 59% of children picked the white doll, whereas when children were asked to identify a doll with negative characteristics such as, “Give me the doll that looks bad,” 59% of children picked the Black doll. From this study, the Clarks concluded that prejudice and segregation caused African-American children to perceive themselves as inferior and carry internalized racism.²⁹ 

Prejudice Following the COVID-19 Pandemic

As we’ve explored, prejudice can impact the quality of care that certain groups receive in healthcare. Unfortunately, health crises can have the added impact of increasing prejudice towards particular groups. Consider the prejudice that gay and bisexual men faced during the HIV/AIDS epidemic in the early 1980s: people created a link between the virus and sexual orientation, causing discrimination against members of the LGBTQ+ community and popularizing the assumption that anyone with the virus must be gay. 

Unfortunately, health epidemics continue to exacerbate prejudice towards particular groups. During the COVID-19 pandemic, there was a rise in violence towards Asian Americans as people came to blame Asians for the outbreak. In 2020, researchers Neeraj Kaushal, Yao Lu, and Xiaoning Huang conducted a study to see how the pandemic impacted perceptions of racial minorities in the workplace. They conducted a national survey in the U.S. where participants were assessed for their attitudes towards East Asian and Hispanic individuals. Participants each read a hypothetical workplace setting that was the same except for the name of the employee, which hinted at their racial group (e.g., Brian Chen would allude to East Asian). After reading the hypothetical story, participants were asked their preference for having the individual as a colleague from 0 to 10 (extremely unlikely to extremely likely). One group of participants was first asked to read a passage about the pandemic and respond to questions about how it affected the employment, earnings, and health of the respondents and their families.

The survey results showed that people who had read the COVID-19 passage exhibited higher levels of prejudice towards both East Asian and Hispanic colleagues, with fewer respondents selecting extremely positive ratings and more selecting extremely negative ratings. This effect was heightened for individuals who lived in areas of the U.S. with higher COVID-19 rates and for people who had lost jobs due to the pandemic. The prejudiced effect was diminished for individuals who lived in areas with a high concentration of Asian residents. The study showed that health crises, especially those with major economic consequences, increase prejudicial attitudes towards minority groups, but also that contact with minorities can reduce prejudice.³⁰ 

Confronting our (Un)conscious Bias with Dr Lasana Harris

The Decision Lab sat down with UCL Professor of Behavioral Science Dr. Lassana Harris to discuss unconscious biases. In this podcast episode, Harris explains that prejudice often evolves from our fight-or-flight response to stimuli that have been deemed dangerous by societal beliefs. Harris also provides guidance for how we can debias, going beyond organization-sanctioned bias trainings (that are too often just for show) to really confront our individual responsibilities. 

Breaking the Silence: Overcoming Stigma in Health Choices Through Group Discussions

Not only is there prejudice towards certain demographic groups in healthcare, but also towards particular health issues as well. Women feel embarrassed to buy tampons, and people feel ashamed to seek mental health support. Due to these stigmas, people are less likely to adopt beneficial health practices. In this article, our writer Dr. Clarrissa Mang explores how stigma, taboos, and social norms impact health choices and provides suggestions on how they can be overcome. 

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