Semmelweis Effect

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What is the Semmelweis Effect?

The Semmelweis effect/reflex is a cognitive bias in which one rejects new information in favor of the information which one already possesses. 

We can find 3 definitions within the wikipedia article. 

“The Semmelweis reflex or “Semmelweis effect” is a metaphor for the reflex-like tendency to reject new evidence or new knowledge because it contradicts established norms, beliefs or paradigms.[1]” 
This is the definition under the first citation, which I found to be an interesting read for the portion that I reviewed 
The next definition is Timothy Leary’s definition of the Semmelweis reflex; “Mob behavior found among primates and larval hominids on undeveloped planets, in which a discovery of important scientific fact is punished.”. One may wish to note the errors in his definition, which can be discussed further in the comment section. 

The third and final definition is by Thomas Szasz. From the wiki: “In section 3 of the preface to the fiftieth anniversary edition of his book The Myth of Mental Illness, Thomas Szasz describes an early exposure to Semmelweis’s life and the reaction to his finding as giving him “a deep sense of the invincible social power of false truths”.[4]”

To reiterate what this effect is one more time, from – the Semmelweis Effect is “the tendency to reject new evidence that contradicts a paradigm.[46]”

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Why is the Semmelweis effect/reflex called what it is called?

We can read @ – that “Dr. Ignaz Semmelweis (1818-1865) discovered that child-bed fever mortality rates reduced ten-fold when doctors washed their hands between patients and, most particularly, after an autopsy. He proposed washing hands between patients as a good practice in 1861, although he was unable to provide a scientific explanation. His hand-washing suggestions were rejected by doctors of his time, interestingly also for non-scientific reasons. For instance, some doctors refused to believe that gen- tlemen’s hands could transmit disease. Semmelweis’s discovery was widely accepted only in the early 1900’s, nearly four decades after his death. Such a lack of acknowledgement of new knowledge is today known as the Semmelweis effect. A metaphor for a certain type of human behavior, the Semmelweis reflex-effect is characterized by rejection of a new knowledge because it contradicts entrenched norms, beliefs and paradigms2.”

For more on Semmelweis’ background, check out the info @

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How can we avoid this cognitive bias?

I would suggest perhaps not jumping to conclusions quickly upon hearing new information. When time allows, analyze situations to gain a clear understanding of what is happening before reacting to them 

All in all, remember that we all may fall prey to faulty thinking. Best to help each other out of it before any serious harm is caused by it.

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