Every second of every day, your brain is guessing. It guesses that the step in front of you is solid, that the chair will hold your weight, and that the car slowing down at the crossing is going to stop. You don't check any of this. You just act.
That's not laziness. It's survival. But the same habit that keeps us moving can also lead us to believe things that aren't true – and when those beliefs are shared, they can turn into "facts" that everyone repeats and nobody checks.
Your brain is a guessing machine (and that's a good thing)
There is far too much going on around us to think carefully about everything. So the brain uses mental shortcuts – quick rules of thumb that give a "good enough" answer fast.
In a 1974 paper, psychologists Amos Tversky and Daniel Kahneman showed that people rely on a small number of these shortcuts when making judgements they can't be sure about. Most of the time they work well. But they also produce errors that are predictable – the same mistakes, made by lots of people, in the same way (Tversky and Kahneman, 1974).
Key insight
Assumptions aren't a flaw in how we think. They're how we cope with a world that moves too fast to check everything. The trouble starts when we forget they're guesses and start treating them as facts.
When shortcuts lead us astray
Here are some everyday shortcuts, how they help, and how they can trip us up.
| Shortcut | How it helps | How it can trip us up |
|---|---|---|
| What springs to mind feels common | Lets us judge risk quickly from what we remember | A scary story we heard once can feel more likely than it really is |
| First numbers stick | Gives us a quick starting point for a guess | The first figure we see can drag our judgement towards it, even if it's wrong |
| Seeing what we expect | Helps us make sense of things fast | We notice what fits our beliefs and skip over what doesn't |
| Familiar feels true | Saves us re-checking things we've met before | A claim we've heard many times starts to feel right – even when it isn't |
The same shortcuts shape our snap judgements about other people too. That's worth remembering when we meet someone whose life, body or background looks different from our own.
How a guess becomes a "fact"
Repetition does the heavy lifting. Researchers have found that hearing a statement again and again makes it easier for the brain to process – and we mistake that ease for truth. In one set of experiments, repeated false statements were rated as more believable even when people already knew the correct answer (Fazio et al., 2015). The researchers gave the belief that vitamin C prevents colds as an example of a myth that may take root this way.
Sharing does the rest. A major study looked at around 126,000 stories shared on Twitter (now X) between 2006 and 2017. False stories spread further, faster and to more people than true ones. The researchers suggested one reason: false stories tended to feel more new and surprising, which made people more likely to pass them on (Vosoughi, Roy and Aral, 2018).
Put those together and you get a loop:
The false fact loop: each time a claim goes round, it feels a little more true.
A real UK example: the MMR scare
In 1998, a paper led by Andrew Wakefield in The Lancet suggested a link between the MMR (measles, mumps and rubella) vaccine and autism. The journal retracted it in February 2010. In 2011, the BMJ concluded it was not just flawed but an elaborate fraud (Godlee, Smith and Marcovitch, 2011).
By then the damage was done. The paper had fuelled a widespread belief that the vaccine causes autism, and fewer children were vaccinated in many countries (Roos, 2011). Today the NHS is clear: research has shown there is no link between the MMR vaccine and autism (NHS, 2024). The NHS also explains that if 95% of children had the vaccine, measles would stop spreading completely (NHS, 2023).
Key insight
A false fact can take minutes to create and decades to undo. And as the research on repetition shows, knowing the facts doesn't fully protect us once a false claim feels familiar.
How to check your own assumptions
You can't switch off your brain's shortcuts – and you wouldn't want to. But you can build in a pause before you believe or share something. UK fact-checking charity Full Fact suggests three simple questions (Milne, 2026):
| Ask yourself | Why it helps |
|---|---|
| Where's it from? | A screenshot or "a friend of a friend" isn't a source. For health claims, check the NHS website or ask your GP or pharmacist. |
| What's missing? | Headlines, numbers and quotes without context can tell half a story. Read the whole thing. |
| How do you feel? | If a post makes you angry or scared, that's often the point. Strong feelings are a signal to slow down, not speed up. |
One more habit worth building: when you catch yourself saying "everyone knows that…", ask how you know it. If the honest answer is "I've just heard it a lot", that's a good moment to check.
The bottom line
Making assumptions is part of being human. It's what lets us get out of bed, cross the road and get on with life. The goal isn't to stop guessing – it's to remember that a guess is a guess, and to check before we pass it on. Every time we pause instead of share, we break the loop.
References
Fazio, L.K., Brashier, N.M., Payne, B.K. and Marsh, E.J. (2015) 'Knowledge does not protect against illusory truth', Journal of Experimental Psychology: General, 144(5), pp. 993–1002. Available at: https://www.apa.org/pubs/journals/features/xge-0000098.pdf (Accessed: 27 September 2026).
Godlee, F., Smith, J. and Marcovitch, H. (2011) 'Wakefield's article linking MMR vaccine and autism was fraudulent', BMJ, 342, c7452. Available at: https://doi.org/10.1136/bmj.c7452 (Accessed: 27 September 2026).
Milne, C. (2026) Getting started with the Misinformation Toolkit. Full Fact, 28 May. Available at: https://fullfact.org/toolkit/getting-started-with-the-misinformation-toolkit/ (Accessed: 27 September 2026).
NHS (2023) Why vaccination is important and the safest way to protect yourself. Available at: https://www.nhs.uk/vaccinations/why-vaccination-is-important-and-the-safest-way-to-protect-yourself/ (Accessed: 27 September 2026).
NHS (2024) MMR (measles, mumps and rubella) vaccine. Available at: https://www.nhs.uk/vaccinations/mmr-vaccine/ (Accessed: 27 September 2026).
Roos, R. (2011) Report says 1998 vaccine-autism study was fraud. CIDRAP News, 6 January. Available at: https://www.cidrap.umn.edu/public-health/report-says-1998-vaccine-autism-study-was-fraud (Accessed: 27 September 2026).
Tversky, A. and Kahneman, D. (1974) 'Judgment under uncertainty: heuristics and biases', Science, 185(4157), pp. 1124–1131. Available at: https://www.science.org/doi/10.1126/science.185.4157.1124 (Accessed: 27 September 2026).
Vosoughi, S., Roy, D. and Aral, S. (2018) 'The spread of true and false news online', Science, 359(6380), pp. 1146–1151. Available at: https://www.science.org/doi/10.1126/science.aap9559 (Accessed: 27 September 2026).
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The content provided in this blog post is for educational and informational purposes only and does not constitute professional medical advice, diagnosis, or treatment. Always seek the advice of your GP, psychiatrist or another qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. Never start, stop or change a prescribed medicine without speaking to your prescriber first.
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