Cognitive biases: the thinking shortcuts that mislead us about health
Cognitive biases are systematic, predictable errors in judgement that come from the mental shortcuts everyone uses to make quick decisions. They are not a sign of low intelligence or mental illness. In health, they explain why a single frightening story can outweigh good evidence, why the first number we hear sticks, why we assume illness will happen to other people, and why a WhatsApp forward that agrees with us feels true. You cannot switch biases off, but you can slow down for important decisions, ask for numbers, look for evidence against your view and check claims with a trusted source.
Health writers · Aslam Clinic
A is a predictable error in judgement that comes from the shortcuts the brain uses to decide quickly. Everyone has them, including doctors, professors and scientists. They are not a sign of stupidity or of a mental health problem; they are the price we pay for a brain that can make thousands of decisions a day without stopping to calculate each one.
Most of the time the shortcuts work. They go wrong in exactly the situations that matter most for health: rare risks, uncertain numbers, frightening stories, and advice that arrives from people we trust. Knowing the common patterns will not make you immune, but it gives you a reason to pause at the right moment.
Where the idea comes from
In 1974 the psychologists Amos Tversky and Daniel Kahneman published a short paper in Science describing three mental shortcuts, or heuristics, that people use when judging uncertain things: how similar something looks to a typical example (representativeness), how easily examples come to mind (availability), and adjusting from a starting number (anchoring). They showed that these shortcuts are usually useful but produce errors that are systematic, meaning they push in the same direction again and again. Kahneman later described two modes of thinking — a fast, automatic, intuitive mode and a slower, effortful, deliberate one. Biases mostly live in the fast mode.
Ten biases that shape health decisions
| Bias | What it is | How it shows up in health |
|---|---|---|
| Confirmation bias | Noticing and trusting information that fits what we already believe | Someone convinced that insulin 'damages the kidneys' remembers every relative on insulin who needed dialysis and ignores that high sugar itself caused the damage |
| Availability | Judging how likely something is by how easily an example comes to mind | One neighbour's bad reaction to a vaccine feels more real than millions of safe doses |
| Anchoring | Sticking too closely to the first number or idea we hear | The first blood pressure reading, or the first diagnosis offered, shapes everything that follows even when later information disagrees |
| Optimism bias | Believing bad things are more likely to happen to other people | "Diabetes runs in my family, but I'll be fine" — so the sugar test keeps being postponed |
| Present bias | Valuing what happens today far more than what happens later | Skipping tablets that prevent a stroke in ten years because they cause a mild side effect this week |
| Sunk cost | Continuing something because of what we have already spent on it | Staying with a treatment that is not working because so much money and time have gone into it |
| Bandwagon effect | Doing or believing something because many others do | Asking for a 'drip' or injection for weakness because everyone in the family gets one |
| Framing | Reacting differently to the same facts depending on how they are worded | "90% survive the operation" feels safer than "10% die", though they mean the same thing |
| Dunning–Kruger effect | Overrating our own skill in areas where we know little | Feeling confident enough to adjust or stop medicines after reading a few posts online |
| Mistaking coincidence for cause | Assuming that because B followed A, A caused B | A cold clears up after a herbal remedy — as most colds do by themselves within a week or two |
What the research on framing and optimism shows
Framing is one of the best-studied biases in medicine. In a 1982 study in the New England Journal of Medicine, patients, students and doctors were asked to choose between surgery and radiotherapy for lung cancer. Their choices shifted depending on whether the same results were presented as the chance of surviving or the chance of dying — and doctors were affected as well as patients. This is why a good clinician gives numbers both ways, and why it is fair for you to ask: "Out of 100 people like me, how many are helped, and how many are harmed?"
Optimism bias is also robust. Reviewing the research in 2011, the neuroscientist Tali Sharot described how people underestimate their chances of illness, accidents and divorce, and update their beliefs more readily after good news than bad. Hope is healthy; optimism bias becomes a problem when it delays a check-up, a test or a change in habits.
The Dunning–Kruger effect: less dramatic than the internet says
In the original 1999 study, Justin Kruger and David Dunning tested students on logic, grammar and humour. Those who scored in the bottom quarter — around the 12th percentile on average — estimated that they were around the 62nd. The best performers slightly underestimated themselves. The authors argued that the skills needed to do a task well are often the same skills needed to judge whether you did it well.
The popular version — a curve with a "Mount Stupid" peak, and the claim that the least able people think they are experts — goes well beyond the data. In the original results, people who performed worst still rated themselves lower than people who performed best; everyone simply bunched towards "a bit above average". Several researchers, including Gignac and Zajenkowski in 2020, have argued that much of the pattern can be produced by statistics alone (regression to the mean plus the general tendency to rate ourselves above average). The debate continues. The fair summary is that people are often poor judges of their own ability, especially in unfamiliar fields — which is a reason for humility all round, not a label to throw at others.
Biases and medicines
Several biases combine around medicines. The means that expecting side effects can make people feel them, and availability makes one relative's bad experience loom large. Present bias makes daily tablets for silent conditions such as easy to drop, because the benefit — a stroke that does not happen — is invisible. Anchoring and bandwagon effects push towards injections and multiple medicines as "stronger" treatment, even when a single tablet is the right choice.
- "This medicine is too strong" is often availability and framing at work. Ask what the actual chance of a serious is, out of 100 or 1,000 people.
- "I feel fine, so I've stopped my tablets" is present bias. Many medicines work by preventing a future problem, not by making you feel better today.
- "I've already spent so much on this treatment" is sunk cost. What matters is whether it is working now, not what it has cost so far.
Misinformation on WhatsApp and social media
Health forwards spread because they exploit biases. They are vivid (availability), they often confirm what people already suspect about doctors or medicines (confirmation bias), they arrive from people we trust, and they are passed on by many others (bandwagon). A voice note from a relative saying that a kitchen remedy cures diabetes carries more emotional weight than a guideline written by strangers.
The World Health Organization uses the word "infodemic" for the flood of true and false information that comes with any health scare, and warns that it causes confusion and risk-taking that harms health. Its advice includes checking who is behind a claim, following links to see whether the source really says what is claimed, and asking yourself why a particular message grabbed you.
How to think more clearly about health decisions
Biases cannot be removed by willpower, and simply knowing their names does little. What helps is building a few habits of slowing down for decisions that matter:
- Ask for numbers, both ways. How many people out of 100 benefit, and how many are harmed? What happens if I do nothing?
- Look for evidence against your first view. Deliberately ask, "What would change my mind?" This is the most direct counter to .
- Separate the story from the statistic. A vivid case is real, but it is one case. Ask how common it is.
- Sleep on big decisions when there is no emergency. The fast, emotional mode of thinking is strongest when you are rushed, frightened or exhausted.
- Get a second opinion from a qualified person, not from the largest number of relatives. Agreement from many people who share the same bias is not independent evidence.
- Notice discomfort. When new information clashes with what you believe, the uneasy feeling — — tempts us to dismiss the information. Treat that feeling as a signal to look more carefully, not less.
Common questions
Still not sure what to do?
Reading about a symptom only goes so far. A doctor who can ask you questions and examine you will get further in ten minutes than any article can.
