Joachim Andersen

Accelerated Learning

The Placebo and Nocebo Effects: How Expectations Heal and Harm

A single white pill in a crystal dish on an old walnut apothecary counter at night, between a glowing amber bottle lit by a candle and a dark smoky bottle, with porcelain jars on shelves behind
The same pill can heal or harm, depending on what we expect.


The Placebo and Nocebo Effects describe how our expectations can change how we feel. When we believe a treatment will help, a sugar pill can ease our pain, and when we expect something to harm us, a harmless pill can make us feel sick. The treatment may be fake, but the effects we feel are real.

The essence of The Placebo and Nocebo Effects is to ask: “How much of what I’m feeling comes from the thing itself, and how much from what I expect it to do?”


Origin: The Wooden Tractors

In 1796, the American physician Elisha Perkins patented his “Tractors”, a pair of pointed metal rods that he claimed could draw pain out of the body when passed over the skin. They sold well in America and Britain, and even George Washington bought a set.

In 1799, the British physician John Haygarth tested them by treating patients with fake tractors made of wood instead of metal. The fake tractors worked just as well as the real ones. In his book Of the Imagination, as a Cause and as a Cure of Disorders of the Body, he concluded that the results showed what a powerful influence “mere imagination” has on disease.

In 1955, the American anesthesiologist Henry Beecher published “The Powerful Placebo”, an influential paper that established the idea that placebo effects are clinically important. In 1961, the physician Walter Kennedy coined the word nocebo, Latin for “I shall harm”, for the opposite of placebo, which means “I shall please”.


Key Concepts

  • Expectations Shape Experience
    In 1985, the psychologist Irving Kirsch suggested that placebos work through what he called response expectancies: when we expect to feel different, we often do. Our brain doesn’t just receive signals from the body, it also predicts them, and those predictions shape what we feel.

  • The Ritual Matters
    The effect doesn’t come from the pill alone, but from everything around it, including the doctor’s confidence, the attention we receive, the price, the packaging and our past experiences with similar treatments.

  • Nocebo Is the Dark Twin
    Warnings, worries and rumors can create real symptoms. When we expect side effects, we start to notice and attribute ordinary aches, tiredness and nausea to the treatment, which can make us stop taking something that would have helped.

  • Real, but Limited
    Placebos mainly affect how we experience symptoms, such as pain, nausea and fatigue. They don’t shrink tumors or cure infections, and much of what looks like a placebo response is simply the natural course of an illness or regression to the mean.


Some Examples

  • The Fake Knee Surgery
    In 2002, Bruce Moseley and colleagues randomly assigned 180 patients with arthritis of the knee to one of two common arthroscopic operations or to fake surgery, where the surgeon only made small cuts in the skin and pretended to operate. For two years, neither the patients nor the people assessing them knew who had received what, and those who had the fake surgery reported just as much improvement in pain and function as those who had the real operations.

  • The Pills Labeled Placebo
    In 2010, Ted Kaptchuk and colleagues gave 80 patients with irritable bowel syndrome either no treatment or a bottle clearly labeled “placebo pills”, and told them openly that the pills contained no medicine. After three weeks, the patients who took the pills reported significantly greater improvement in their symptoms, suggesting that placebos can sometimes work even without deception.

  • The Expensive Painkiller
    In 2008, Rebecca Waber, Baba Shiv, Ziv Carmon and Dan Ariely gave 82 volunteers mild electric shocks before and after taking a “new painkiller” that was actually a placebo. When told the pill cost $2.50 per dose, 85% reported less pain, but when told it had been discounted to 10 cents, only 61% did.

  • The Statin Side Effects
    In the SAMSON trial, published in 2020, Frances Wood and colleagues at Imperial College London gave 60 people who had stopped taking statins because of side effects twelve monthly bottles in random order: four with a statin, four with a placebo and four empty. Their symptoms were about twice as strong in the months with tablets as in the empty months, but 90% of the symptoms they felt with the statin also appeared with the placebo, and half of them went back to taking statins after the trial.


Applying The Placebo and Nocebo Effects

When I want to use this mental model, this is the simple process I follow:

  1. Ask What I Expected
    When something makes me feel better or worse, I ask what I expected it to do before I tried it, and whether that expectation could explain what I’m feeling.

  2. Look for a Fair Comparison
    Before I trust a supplement, a treatment or a new routine, I look for evidence from trials that compare it with a placebo, because simply feeling better after trying something proves very little.

  3. Watch My Words
    Because what I say shapes what others expect, I try to describe treatments, plans and changes honestly, without either overselling them or filling people with fear.

  4. Be Careful With Warnings
    When I read a long list of side effects, I remember that many of them also appear in people taking a placebo, and I talk to a doctor before stopping something that helps me.

  5. Use the Ritual
    I make the routines that help me, like exercise, sleep or meditation, pleasant and meaningful, because belief and ritual can add to the real benefits.

  6. Separate Feeling From Fact
    When I want to know whether something really works, I measure outcomes that don’t depend on how I feel, rather than relying on my impressions alone.


Pitfalls and Limitations

  • The effect is often overstated. In 2001, Asbjørn Hróbjartsson and Peter Gøtzsche reviewed 114 trials that compared placebo with no treatment at all, and found little evidence of large effects, except for small benefits on pain and other subjective outcomes.

  • Not everything is a placebo. Many people improve simply because their illness gets better on its own, and confusing that with a placebo effect can lead to the wrong conclusions.

  • It can be exploited. Quacks and sellers of useless remedies rely on placebo responses and testimonials, which can make people delay treatments that actually work.

  • Deception has a cost. Giving people fake treatments without their consent can damage trust, which is why doctors and researchers have to be careful about how they use placebos.


Related Mental Models

The Placebo and Nocebo Effects remind me that my mind is part of every treatment, decision and experience, and that what I expect can heal or harm. By asking what I expected, looking for fair comparisons and choosing my words with care, I can benefit from the power of expectations without being fooled by it.



To learn more about mental models and philosophy,
check out: Wise: How to Attain a Deep Understanding of the World, History, Our Place in It, and How to Live Well



📚 Sources and References