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]

The term derives from the name of Ignaz Semmelweis, a Hungarian physician who discovered in 1847 that childbed fever mortality rates fell ten-fold when doctors disinfected their hands with a chlorine solution before moving from one patient to another, or, most particularly, after an autopsy. (At one of the two maternity wards at the university hospital where Semmelweis worked, physicians performed autopsies on every deceased patient.) Semmelweis's procedure saved many lives by stopping the ongoing contamination of patients (mostly pregnant women) with what he termed "cadaverous particles", twenty years before germ theory was discovered.^[2] Despite the overwhelming empirical evidence, his fellow doctors rejected his hand-washing suggestions, often for non-medical reasons. For instance, some doctors refused to believe that a gentleman's hands could transmit disease.^[3]

While there is uncertainty regarding its origin and generally accepted use, the expression "Semmelweis Reflex" had been used by the author Robert Anton Wilson.^[4] In Wilson's book The Game of Life, Timothy Leary provided the following polemical 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".^{[citation needed]}

In the preface to the fiftieth anniversary edition of his book The Myth of Mental Illness, Thomas Szasz says that Semmelweis's biography impressed upon him at a young age, a "deep sense of the invincible social power of false truths."^[5]

Confirmation bias is the tendency to favour information that is consistent with prior beliefs or values.^[6] When Semmelweis introduced the handwashing proposal, the existing beliefs on disease transmission that other doctors held at that time included miasma theory, which suggests diseases were spread through “bad air”. It was also a common belief that childbed fever happens due to factors like inherent weakness of the patients rather than unclean hands. The handwashing proposal contradicted existing beliefs, which may have led people to be biased against accepting the proposal, despite empirical evidence showing that handwashing significantly reduces maternal mortality rates from 18% to less than 3%.^[7]

Authority bias reveals people are more likely to be influenced by the opinions of authority figures. In the days before the medical professions made the connection between germs and disease, senior doctors, including Semmelweis’ professor Johann Klein, were scornful of Semmelweis' idea of preventing bacterial infections through antimicrobial strategies that are now widely accepted.^[8] The leading obstetrician, Charles Meigs, was firmly against Semmelweis’s doctrine because “doctors are gentlemen, and gentlemen’s hands are clean.”^[9] Throughout human history, obeying authority figures often give better a means of survival because they normally have greater access to resources at the top of the social hierarchy.^[10] As a result, although the authority figures can be wrong, the medical community tends to believe them rather than Ignaz Semmelweis, a professor assistant at that time.

The Semmelweis reflex also exemplifies how belief perseverance causes individuals to adhere to their initial beliefs despite contradicting evidence. The human brain has fully developed the cerebral cortex and the prefrontal cortex (PFC), which equips individuals with the power to resist primitive instincts and adaptability but also maintains the status quo and avoids deliberate changes.^[11] Therefore, belief perseverance can be interpreted as a coping mechanism that reflects the human tendency to resist change and discomfort.^[12]

The Semmelweis proposal was met with unanimous rejection and hostility from the medical community in the 19th century, exemplifying the phenomenon of groupthink, where consensus overrides consideration of alternatives.^[13] There may have been pressure to conform to the common beliefs, hindering individuals from accepting Semmelweis's innovative idea. In an open letter, Semmelweis slammed other doctors as “ignorant murderers”, which only served to further isolate him as an outlier from the group.^[8] Research on barriers to the transmission of new ideas highlights the challenge of adopting innovative concepts, especially when they are perceived as superior by external entities, as this could pose a threat to the collective pride of the group.^[14]

In the book Thinking, Fast and Slow, Daniel Kahneman used the term “theory-induced blindness” to explain how a false theory survived for so long.^[15] When people accept a theory, System 1 internalised it as a tool for thinking, making it difficult to realise any potential flaws. Even after discovering that the theory doesn't explain the model well, system 1 automatically assumes that there must be a way to explain it but may not look deeper into what that explanation is. On the other hand, discarding an inherent theory is difficult because it requires the deliberate involvement of system 2.

Semmelweis reflex is often seen as an age-old bias, but it persists in modern times, as illustrated by the delayed recognition of COVID-19's airborne transmission. Despite some evidence indicating aerosol spread, the focus of WHO was primarily on droplet transmission because almost all infectious diseases are spread through droplets. It wasn’t until December 2021 that the WHO officially recognised airborne transmission, which shows the challenge of shifting entrenched beliefs, especially when the prevailing understanding aligns with established norms. Integrating innovative perspectives swiftly in existing frameworks poses a significant challenge. As the epidemiologist Christopher Dye says, “What the WHO says is normally based on a consensus of expert advice and opinion.”^[16]

To mitigate the Semmelweis reflex, one needs to critically evaluate beliefs that are taken for granted, which requires the deliberate engagement of system 2 thinking. Research examining dual-process interventions in diagnostic reasoning shows cognitive forcing tools and guided reflection can enhance diagnostic accuracy.^[17] These interventions encourage individuals to actively consider alternative diagnoses that may not be intuitive, thereby enabling them to consciously confront potential biases. However, conflicting findings from other studies suggest that these strategies might not consistently yield the desired results, particularly among students and young doctors.^[18][19]

Most research on the Semmelweis reflex primarily focuses on its historical origins and implications in medical and healthcare settings, particularly in diagnosis. However, the reluctance to embrace new ideas is not limited to medical professionals; it can also hinder progress and innovation within all walks of life. Research on the effects of Semmelweis reflex in different fields is therefore needed to develop more applicable interventions.