Optimism bias is the tendency to overestimate the likelihood of positive events happening to you while underestimating the likelihood of negative ones. Psychologist Tali Sharot, who popularized the term, defines it as “the inclination to overestimate the likelihood of encountering positive events in the future and to underestimate the likelihood of experiencing negative events.” It affects an estimated 80% of the population and shapes everything from how you plan a home renovation to whether you bother getting a flu shot.
This isn’t the same as being a generally cheerful person. Optimism bias is a specific cognitive error: your brain processes good news and bad news differently, and the result is a distorted picture of what’s likely to happen to you.
How Your Brain Filters Good and Bad News
The core mechanism behind optimism bias is asymmetric belief updating. When you receive information that’s better than expected (your risk of a disease is lower than you thought), you readily adjust your beliefs. When you receive information that’s worse than expected (your risk is higher than you thought), you barely budge. This isn’t a conscious choice. It happens automatically, at the level of how your brain processes new information.
Brain imaging studies have identified two regions closely tied to this pattern. The anterior cingulate cortex, which helps you imagine the future and process self-relevant information, shows higher activity in people with stronger optimistic tendencies, particularly when they picture positive future events. The inferior frontal gyrus, involved in processing relevant cues and inhibiting responses, tracks with how much people update their beliefs in the classic belief-updating task. When people imagine negative future events, activity in both the anterior cingulate cortex and the amygdala (the brain’s threat-detection center) drops compared to when they imagine positive ones. In other words, the brain literally dampens its response to bad news about the future.
Optimism Bias vs. Being an Optimist
These two concepts overlap in everyday conversation but are scientifically distinct. Dispositional optimism is a personality trait: a general expectation that things will work out. There’s no claim about accuracy. You can be a dispositional optimist and still be correct about your future prospects.
Optimism bias, by contrast, is defined by its inaccuracy. It’s measured against specific risks (your chance of getting cancer, your likelihood of divorce) and, by definition, involves underestimating those risks. Research on smokers illustrates the difference clearly: a smoker can be a generally optimistic person while also holding an unrealistically low estimate of their personal lung cancer risk. Studies have found that these two forms of optimism don’t necessarily go hand in hand. Being a generally positive person doesn’t automatically make you more likely to underestimate specific health risks.
Real Consequences for Health and Safety
Optimism bias has measurable effects on behavior, particularly around health. Because the bias lowers your perception of personal risk, it reduces the anxiety and concern that would normally motivate you to take precautions. Research during the COVID-19 pandemic found that people with stronger optimism bias perceived themselves as less likely to get infected and viewed the virus as less serious. That lower risk perception made them less anxious, which in turn made them less likely to seek out prevention information or engage in protective behaviors.
This pattern repeats across health contexts. During the H1N1 influenza outbreak, unrealistic optimists perceived themselves as less likely to become infected and were less likely to take flu prevention steps. Studies on smoking have found that optimistically biased smokers were less interested in quitting because they underestimated their personal risk. College students who underestimated their health risks showed less interest in risk information and were less likely to revise their perceptions even when presented with accurate data.
The chain is consistent: optimism bias lowers perceived risk, which lowers concern, which lowers the motivation to act. This makes it a significant factor in public health, where getting people to take preventive action depends on them first believing they’re actually at risk.
The Bias Gets Stronger With Age
Optimism bias isn’t fixed across your lifespan. Healthy older adults show a stronger version of it than younger adults, and the reason is specific: older adults update their beliefs even less than younger people when faced with undesirable information, while both age groups update equally in response to desirable information. The result is a wider gap between how good news and bad news are absorbed.
This finding held even after researchers controlled for overall memory decline and subjective well-being ratings. It wasn’t that older adults forgot the bad news or were simply happier people. They specifically resisted incorporating negative information into their expectations about the future. This may partly explain a well-documented paradox in aging research: older adults consistently report greater well-being and life satisfaction than younger people, despite facing objectively greater health risks and losses.
What Happens When the Bias Disappears
One of the more striking findings in this area comes from research on depression. People with mild to moderate depression make more accurate judgments across a variety of domains. This phenomenon, known as depressive realism, is essentially the flip side of optimism bias.
In belief-updating tasks, people with major depressive disorder show no optimistic bias when processing information about the likelihood of unpleasant life events. They update their beliefs roughly equally for good and bad news, which produces more accurate estimates. Non-depressed individuals, by comparison, hold overly optimistic beliefs about themselves and their future prospects. The implication is uncomfortable: a certain degree of inaccuracy about your own future appears to be part of normal psychological functioning, and losing that distortion is associated with depression rather than with clear-eyed wisdom.
This doesn’t mean depression grants superior judgment in all contexts. Depressive realism applies most strongly to self-related information and estimates of present circumstances, rather than predictions about others or abstract scenarios.
How to Counteract It
Because optimism bias operates below conscious awareness, simply knowing about it doesn’t make it go away. But structured techniques can reduce its influence, particularly in planning and decision-making.
The most well-established method is reference class forecasting, developed by researcher Bent Flyvbjerg and endorsed by Nobel laureate Daniel Kahneman. Instead of estimating how long a project will take or how much it will cost based on your internal sense of how things will go (the “inside view”), you look at how similar projects actually performed in the past (the “outside view”). If the average kitchen renovation takes 12 weeks and goes 20% over budget, that base rate is a better predictor than your feeling that yours will be different.
A premortem is another practical tool. Before starting a project or making a major decision, you imagine that it has already failed and work backward to identify what went wrong. This forces your brain to generate concrete reasons for failure, which counteracts the bias toward assuming everything will work out. The key principle behind both techniques is the same: replace your intuitive forecast with actual data from comparable situations, and deliberately make space for negative outcomes in your planning.