Placebo effect, usually the pills that patients believe them as treatments but are control treatment, in the researches is shown to be effective in reducing symptoms associated with the diseases, but a study done by Harvard researchers Kaptchuk, et al. (2010) showed that placebos even without deception to the patients worked very effectively in reducing symptoms associated with an Irritable Bowel Syndrome. It’s such a double mystery of why the fake pills that participants are fully aware work in reducing symptoms.

Why does placebo effects exist in humans in the first place? In other words, what is the evolutionary basis of placebo effect? I just read this wonderful article on by Steinkopf (2015) called The Signaling Theory of Symptoms: An Evolutionary Explanation of the Placebo Effect that explains the mystery and gives further specific hypotheses that could be tested out. The key theory proposed is called Signaling Theory of Symptoms (STS). I will explain and summarize the main points from the article Steinkopf (2015).

  1. Ultimate functions of discernable signals of the disease serve defense and healing purposes.

According to Wall (1999), pain informs that sufferer of tissue damage and motivates appropriate action to take. The placebo effect in such cases can be taken as a small aspect of adaptation in motivating us to do something. Wall (1999) proposed that as one has taken “appropriate action,” the need state has been fulfilled and the pain diminishes. Just like, when you touch the hot stove and your body feels pain. But the pain goes away as you move yourself away from the stove. Or consider a time you shiver when you are cold; shivering warms your body. Or consider coughing and sneezing; the acts expel pathogens. Or consider the example of fever, which creates an uncomfortable situation for pathogens. These discernible symptoms of immune reactions are not caused by the infection itself but by your bodily response to the infection (Steinkopf, 2015).

  1. Another ultimate mechanism of such discernable symptoms is to demand help and care from potential helpers.

STS, like any other evolutionary theories, is that our human mind is adapted to Paleolithic age, not modern circumstances yet (Nesse & Williams, 1995; Wilson, 1978). Though it is not known how medical treatment was in the Paleolithic age, any kind of treatments would involve familiar group members. Social support is extremely important for survival; even in contemporary hunter-gatherer societies, injuries and sickness keep the group members from hunting and gathering but sick group members are provided with food (Gurven et al., 2000). The seemingly altruistic act by “healthy” group members to the sick could be explained through delayed reciprocity and social reputation (Barclay, 2012, 2013). Secondly, medications and rituals would be involved, though changed over time and differed between groups. Differential cultures differed between tribes and over time, have different results, and humans have been selected for flexible treatment demands depending on the medical treatments they received. Hence Steinkopf (2015) proposed that social support (Benedetti, 2013) and patient’s general expectation (Stewart-Williams & Podd, 2004) trigger the placebo effect, reducing the symptoms or cues in sufferer as the help or social support has been received. For example, a young infant’s cry is both a cue for a need and an exaggerated signal to extract more resources from the parents than the parents would give in (Trivers, 1974). The infant might stop crying when the needs are fulfilled.

A very interesting study (Hashish, et al., 1988) used an “off” ultrasound device and showed that placebo effect occurred when they told participants that the device reduced the swellings caused by dental treatment. However, the swelling was not reduced when the subject used the machine alone. Another study showed that the nasal intake of oxytocin, one of the main hormones in increasing trust, improved the analgesic effect of a placebo ointment, applied to the forearm (Kessner et al., 2013).

Likewise, Valentini et al. (2014) showed that different facial expressions to the participants changed the placebo effect. A sad face enhanced placebo effect, and a happy face enhanced even more effective placebo effect. A sad face indicates that a potential helper empathizes while a happy face may be seen as a sign that the help is very possible from the happy person (Steinkopf, 2015).

  1. Hypotheses and reasoning Steinkopf (2015) put forward are explained.

Hypothesis 1: Patients should show stronger symptoms in a social environment as compared to in isolation.

If symptoms signal a need for social support, they should be more common when the sick is with people than without people.

Hypothesis 2: When other people acknowledge the status of the sick individual and start helping, then the symptoms should diminish.

The signals should be gone when the needs of social support, associated with the signals are fulfilled.

Hypothesis 3: The more sick individuals compete for help and the more scarce potential helpers are, the stronger the expression of symptoms of sick individuals should be.

Competition for the recipient of social support among the sick, might exaggerate the signaling cues.

Hypothesis 4: The aforementioned hypotheses (Hypotheses 1–3) should only be valid for easily discernible symptoms but not for indiscernible aspects of the immune reaction.

The key point is that the cues should be noticeable, for example while coughs and sneezing are easily discernible while concentration of antibodies are not to the potential helpers. However, symptoms and diseases are interrelated; placebo effects have been associated with stomach ulcers, in which swelling and redness are not visible but sickness behaviors such as apathy, tiredness and loss of sexual interests are.

Hypothesis 5: The placebo effect should rather be found in symptoms that do not elicit disgust in potential helpers.

There is also a cost with social support. So the cues that the disease is contiguous while signaling the social support would be so effective in soliciting help from the potential helper.

Hypothesis 6a: When helpers are distant, the sufferer should have strong symptoms that are recognizable from far, such as noises like sneezing, coughing, and cries of pain.

Hypothesis 6b: When helpers are close, the sufferer should not have symptoms that raise the risk of infection of potential helpers, such as coughing and sneezing. Instead, the sufferer should have strong symptoms, such as fever, that are recognizable from close proximity and that do not raise the risk of infection.

When the distance is far from the potential helper, cues like those mentioned in 6a should be more common while the distance is nearer between the potential helper and the sick, the “contagious” symptoms should be suppressed.

Hypothesis 7: Signaling symptoms should only be found in species, where care of the sick can be found.

The cues for social support only make sense when the social support is available.

Hypothesis 8: In species that show maternal care, but not care for sick adults, signaling symptoms should be found in young animals but not in adults.

This is a comparative hypothesis for species where maternal care is available only to the young.


Placebo effects occur when the person’s symptoms are acknowledged and treated with care and support from another person. The signaling function is fulfilled and the symptoms are pacified. Now, before we get to the answer of the question asked in the article, I want to tell you about my story.

When I was on a trip to monument Valley in Utah, I got scratched by dogs in the valley. I was extremely anxious and upset because I was thinking about potential rabies, which could be fatal. My immune system was kicking off; I felt like I had fever and headache. I went to the Emergency Room at the hospital. The doctor happily looked at my small scratch and then soothed me that I was completely fine and discharged me from the hospital. I was happy and the signals went away.

Well, I think this is how spiritual healers work too. When they treat the patient seriously with the patients’ complaints, as long as they earn money from the patient. When the signaling function is completed, the symptoms become pacified. Voila! Magic has happened.

Acknowledgement: Evolutionary theory is a wonderful tool in explaining observed mentality and behaviors in human. Natural selection did not select humans to understand all these evolutionarily forces. Special thanks to the collective insights from (Steinkopf, 2015) and all other researchers, I am better able to understand the phenomenon via reading. Please refer to original insights and detailed elaboration at


Barclay, P. (2012). Harnessing the power of reputation: Strengths and limits for promoting cooperative behaviors. Evolutionary Psychology, 10, 868–883.

Barclay, P. (2013). Strategies for cooperation in biological markets, especially for humans. Evolution and Human Behavior, 34, 164–175.

Benedetti, F. (2013). Placebo and the new physiology of the doctor– patient relationship. Physiological Reviews, 93, 1207–1246.

Gurven, M., Allen-Arave, W., Hill, K., & Hurtado, M. (2000). ‘‘It’s a wonderful life’’: Signaling generosity among the Ache of Paraguay. Evolution and Human Behavior, 21, 263–282.

Hashish, I., Hai, H. K., Harvey, W., Feinmann, C., & Harris, M. (1988). Reduction of postoperative pain and swelling by ultra- sound treatment: A placebo effect. Pain, 33, 303–311.

Kaptchuk T. J, Friedlander E, Kelley, J. M., Sanchez, M. N., Kokkotou, E., Singer, J. P., Kowalczykowski, M., Miller, F. G., Kirsch, I., & Lembo, A. J. (2010) Placebos without deception: A randomized controlled trial in irritable bowel syndrome. PLoS ONE 5(12): e15591. doi:10.1371/journal.pone.0015591

Kessner, S., Sprenger, C., Wrobel, N., Wiech, K., & Bingel, U. (2013). Effect of oxytocin on Placebo analgesia: A randomized study. JAMA, 310, 1733–1735.

Nesse, R. M., & Williams, G. C. (1995). Why we get sick: The new science of Darwinian medicine. New York, NY: Times Book.

Steinkopf, L. (2015). The signaling theory of symptoms: An evolutionary explanation of the placebo effect. Evolutionary Psychology, 13, 1-12.

Stewart-Williams, S., & Podd, J. (2004). The Placebo effect: Dissolving the expectancy versus conditioning debate. Psychological Bulletin, 130, 324–340.

Trivers, R. L. (1974). Parent-offspring conflict. American Zoologist, 14, 249–264.

Valentini, E., Martini, M., Lee, M., Aglioti, S. M., & Iannetti, G. (2014). Seeing facial expressions enhances placebo analgesia. Pain, 155, 666–673.

Wall, P. (1999). Pain: The science of suffering. London, England: Weidenfeld & Nicolson.

Wilson, E. (1978). On human nature. Cambridge, MA: Harvard University Press.


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