Why We Reject New Evidence That Could Directly Help Us: The Semmelweis Effect
Welcome back to PsyberSpace. I'm your host, Leslie Poston. And this week, we're talking about one of the oldest patterns in medicine and one of the most persistent patterns in human thinking. Our tendency to reject evidence that could help us, specifically because it threatens what we already believe. This pattern has a name.
Leslie Poston:It's called the Semmelweis effect, and it's named after a doctor whose story is one of the most frustrating in the history of medical science. But I don't want to just tell you about a dead doctor from the 1800s. I want to connect his story to something happening right now, something that might be making this pattern harder for us to overcome at exactly the moment we need to overcome it the most. There's emerging evidence that a biological factor could be amplifying a bias that was already one of the biggest obstacles to medical progress. Let's get into it.
Leslie Poston:Ignaz Semmelweis was an Austrian Hungarian physician who worked in a Vienna maternity hospital in the 1840s. The hospital had two clinics. One was staffed by doctors and medical students, and the other was staffed by midwives. The death rate from childbed fever in the doctor's clinic was roughly five times higher than in the midwives clinic. Women knew this and would beg not to be admitted to the doctor's clinic.
Leslie Poston:Some preferred to give birth in the street. Semmelweis started investigating why that was. The turning point came for him when a colleague died after being accidentally cut with a scalpel during an autopsy. His symptoms looked almost identical to childbed fever. Semmelweis made the connection that doctors were going directly from performing autopsies to delivering babies without washing their hands.
Leslie Poston:These doctors were carrying something from the dead bodies to the living mothers. He introduced a protocol requiring doctors to wash their hands with a chlorine solution between autopsies and patient care. And the mortality rate in his clinic dropped roughly tenfold. It was one of the clearest cause and effect demonstrations in the history of medicine. That didn't matter.
Leslie Poston:His colleagues rejected it. Not because his data was weak, because his data was excellent. They rejected it because the implication felt unbearable to them. If Semmelweis was right, it meant that doctors had been killing their own patients. And it meant that the hands of a quote gentleman, a trained physician, were a vector for disease.
Leslie Poston:That was an identity level threat, not just an intellectual one. And so the medical establishment of the time pushed back hard. Semmelweis struggled to communicate his findings in a way that didn't alienate people as well, which made things worse, but the communication problem was secondary to the psychological one. The evidence required accepting something that felt like an accusation, and the other doctors couldn't do it. Semmelweis was eventually committed to an asylum, where he died in 1865.
Leslie Poston:It took another two decades before his germ theory was broadly accepted and handwashing became standard practice. In the meantime, an unknowable number of women died from a disease that was already preventable. The term Semmelweis effect, or sometimes Semmelweis reflex, now describes a broader psychological pattern: the automatic reflexive rejection of new evidence because it contradicts established norms, beliefs, ways of doing things. It's recognized as a distinct form of cognitive bias, though it draws on several related psychological mechanisms working together. The most obvious one we've talked about before is cognitive dissonance.
Leslie Poston:That's when new information conflicts with something you believe, especially something you've built your career or your identity around. The mental discomfort is real, and your brain has two ways to resolve it. It can update the belief to accommodate the new evidence, which is hard and sometimes painful, and requires you to rethink decisions you've already made based on the old belief. Or your brain can reject the new evidence, which is fast and feels like relief. A lot of the time the path of least resistance wins.
Leslie Poston:Pessinger's original research on this showed that people will go to remarkable lengths to protect a belief that's central to their self-concept, including dismissing evidence that would be persuasive in any other context. What makes this especially potent in professional settings is that careers are built on paradigms. A surgeon who's performed a procedure a certain way for fifteen years isn't just being asked to learn a new technique. They're being asked to consider the possibility that the old way was causing avoidable harm, which is a much heavier thing to accept. Status quo bias compounds this.
Leslie Poston:Established paradigms aren't just ideas floating around they have institutional momentum behind them. Funding structures, career paths, professional hierarchies, social networks, entire systems, training and credentialing. Changing a paradigm means changing all of that infrastructure, which is expensive and disruptive. And there's also identity protective cognition, which is what happens when a belief becomes fused with your sense of who you are. The doctors in Semmelweis' era weren't just weighing evidence about hand hygiene.
Leslie Poston:They were being asked to accept that their professional identity, the very thing that gave them status and authority, was now associated with causing harm. And that's a much different ask than evaluating a neutral dataset. It's important to distinguish the Semmelweis effect from confirmation bias because they do get confused. Confirmation bias is about seeking. You looking for information that supports what you already think.
Leslie Poston:The Semmelweis effect is about rejecting. You encounter strong disconfirming evidence and push it away, sometimes aggressively. They're related, but the Semmelweis effect has a defensive quality that goes beyond selective attention. It's closer to what happens when an animal is cornered. The response is reactive and sometimes hostile, which is why Semmelweis himself faced professional attacks, not just polite disagreement.
Leslie Poston:Note also that the label can be misused. Not every instance of skepticism toward a new claim is the Semmelweis effect. Sometimes new data really is flawed or a theory doesn't have enough support to justify scrapping established practice. The Semmelweis effect specifically describes cases where the evidence is strong and the rejection is driven by the psychological discomfort or identity threat rather than legitimate methodological concerns. If we called every disagreement a Semmelweis reflex, the concept would lose its diagnostic usefulness.
Leslie Poston:I want to connect this historical pattern to something happening in real time. There's a growing body of research showing that COVID-nineteen infection causes measurable cognitive decline, and the specific cognitive functions it damages are the same ones we need to overcome the Semmelweis effect. Let's look at some of the research. A large community based study in The UK, published in the New England Journal of Medicine, tested over 112,000 participants and found cognitive deficits following COVID infection across the board. People with mild infections who felt fully recovered still showed deficits equivalent to roughly a three point IQ loss.
Leslie Poston:People with unresolved long COVID symptoms showed the equivalent of about a six point loss. People who had been in intensive care for COVID showed about a nine point loss. And here's why that matters for what we're talking about today: reinfection added roughly another two points of cognitive deficit on top of the initial decline. The damage appears to be cumulative. The cognitive domains most affected were memory, reasoning, and executive function, which includes things like planning, weighing competing information, and making decisions that require sustained mental effort.
Leslie Poston:COVID vaccination offered a small but measurable protective effect. A separate study, using objective cognitive testing, found that people who considered themselves fully recovered still performed measurably worse than people who had never been infected. And their self assessments of their own cognition didn't correlate with their actual test results, meaning they didn't realize they lost cognitive ability until the data in the tests they took showed them. Brain imaging research has found physical changes to back this up. There's measurable shrinkage in the hippocampus, which is critical for memory.
Leslie Poston:There's reduced connectivity between the amygdala and the regions that handle emotional regulation and cognition. A twenty twenty five study from a Japanese research group found that people with long COVID brain fog had elevated levels of AMPA receptors throughout the brain. Those are proteins involved in neural signaling. This suggests the brain's communication systems are being disrupted at a structural level. The Alzheimer's Association funded a global research network that found older adults had double the risk of moderate to severe cognitive impairment after COVID compared to younger adults, with people 60 and those who'd had severe infections being especially vulnerable to this.
Leslie Poston:This appears to involve some combination of direct viral damage to the nervous system, widespread inflammation, microvascular damage, and immune system dysfunction in combination. This creates a feedback loop. Overcoming the Semmelweis effect requires exactly the cognitive capacities that COVID infection appears to degrade: executive function, reasoning ability, the capacity to hold conflicting information in the mind and evaluate it carefully, the mental energy to update a belief rather than defaulting to the easier path of rejecting the new evidence. If repeated COVID infections are quietly reducing these capacities across large portions of the population, it becomes incrementally harder for people to engage with new medical evidence in the effortful way that belief updating requires. And if diminished reasoning capacity makes people more susceptible to reflexive rejection of evidence, that includes evidence about COVID's own cognitive harms, which leads to less precaution, more reinfections, and more cumulative damage to the very functions you need to break the cycle.
Leslie Poston:Each turn of that loop makes the next turn a little more likely. And that's not hypothetical. Those are documented deficits in documented cognitive domains with documented cumulative effects from documented reinfections. The loop is in the data. There's yet another layer to all of this.
Leslie Poston:And it has to do with how long it takes for new medical knowledge to actually reach people it could help. Even in the best case scenario where the Semmelweis effect isn't in play and everyone is acting in good faith, there's a structural delay between when something is discovered and when it becomes standard practice. One widely cited study found that the average time from the discovery of new medical knowledge to its use in routine clinical practice is about seventeen years. For new drugs and new medical devices, it's closer to twenty one years. Other analyses have found even wider ranges depending on the type of intervention, from eighteen years for certain psychiatric treatments to fifty four years for smoking reduction strategies.
Leslie Poston:Even more conservative estimates put the average at about nine years from human research to guideline inclusion, with roughly two to three of those years already lost in the gap between when a study is published and when clinical guidelines are updated to reflect it. That gap already has real consequences. Every year of delay is a year where real patients aren't benefiting from new knowledge that already exists. Some of that delay is necessary and beneficial. You obviously want safety testing.
Leslie Poston:You want replication of the studies. You want to make sure an intervention works across different populations before you roll it out everywhere. But a meaningful portion of the delay reflects institutional versions of the Semmelweis effect operating at scale. Medical training creates strong paradigms, and physicians who train under one paradigm are naturally resistant to information that challenges it. Clinical guidelines are updated through consensus processes that were designed to be conservative.
Leslie Poston:Insurance and reimbursement structures are built around established procedures, which creates financial disincentives for adopting new ones. And the organizations that would need to change are often the same ones that need to acknowledge that previous practice was suboptimal. This triggers the same identity protective dynamics that we see at the individual level. On top of all of this, public trust in science has become complicated in ways that make the translation problem worse. The data here is a little contradictory, depending on how you measure it.
Leslie Poston:A 2025 survey found that only eight percent of U. S. Adults reported great trust in science, a decline of nearly 24 percentage points since 2023. But the Pew Research Center's 2025 survey found that 77% of Americans still have at least a fair amount of confidence in science, roughly stable from the prior year. Both surveys agree that trust has become sharply divided along partisan lines, with Democrat and left leaning respondents at roughly 90% confidence and Republicans at about 65.
Leslie Poston:A separate study looked at international data and found that while trust is polarizing in some countries, particularly in The United States, there isn't strong evidence of a universal collapse in trust globally. The pattern is more fragmentation than a wholesale decline. But what all of this means taken together is that we have three separate breaking forces on the adoption of new medical evidence, and they compound each other. The institutional translation gap means knowledge moves slowly from lab to clinic, even under the best conditions. The erosion and polarization of public trust means that even when knowledge does reach the public, a significant portion of people might not act on it.
Leslie Poston:And if widespread cognitive decline from a pandemic is reducing the population's capacity for the kind of effortful, deliberate thinking that belief updating requires, you've added a biological break on top of the institutional and social ones. Each of these is a problem on its own. Together, they create conditions where the gap between what we know and what we do about it could widen rather than narrow at exactly the moment in time when the stakes are highest. I want to end with something practical, like I always do, because I think the psychology of this actually points towards useful responses, even though the picture is very sobering. Awareness of the Semmelweis effect does seem to offer some protection against it.
Leslie Poston:Research on cognitive biases more broadly suggests that people who know about a specific bias and understand how it operates are somewhat better at catching it in themselves. That's not a silver bullet because knowing about a bias doesn't eliminate it. But it does introduce a pause. If you notice yourself having a strong negative reaction to new evidence, particularly evidence that challenges something connected to your professional identity or your sense of how the world works, That reaction is data. It doesn't mean the evidence is right, but it means your evaluation of the evidence might not be as dispassionate as you feel in the moment.
Leslie Poston:On the cognitive side, the research on cumulative damage from COVID reinfection suggests that protecting cognitive function has implications beyond individual health. If each infection incrementally degrades the reasoning and executive function capacities that society needs to evaluate evidence and adapt to new information, that infection prevention has a cognitive dimension that hasn't been part of the most public health messaging. That's a framing I'd personally like to see more researchers and communicators engage with, because the individual health argument for avoiding infection just hasn't been enough on its own to change behavior at scale. And the collective cognition argument might add something that's been missing from the conversation. The seventeen year translation gap is a systemic problem that requires a systemic fix.
Leslie Poston:But there are people working on it. Living guideline models, which update clinical recommendations continuously as new evidence comes in instead of waiting for periodic revision cycles, are one promising approach. Better digital infrastructure for evidence synthesis could reduce the two- three year bottleneck between study publication and guideline updates as well. These are boring process level interventions, but they're the kind of thing that can shave years off the time between when we know something new and when that knowledge helps patients. And regarding trust: The research is fairly consistent that transparency works better than persuasion.
Leslie Poston:People respond to institutions that communicate openly about uncertainty, that acknowledge what they don't know alongside what they do, and are honest when they get something wrong. The fracturing of trust along political lines makes that harder in practice, but the underlying finding still holds. The path toward trust isn't convincing people that science is always right. It's demonstrating that the process is honest and self correcting, which it is when it's working well. Semmelweis's colleagues weren't stupid people.
Leslie Poston:They were trained physicians operating within a system that reinforced their existing understanding. And they encountered evidence that asked them to accept something deeply uncomfortable about their own role in the deaths they were trying to prevent. The psychological forces that made them reject that evidence in 1847 are the same ones operating today in every field and in all of us. What concerns me about this moment is that the biological capacity to resist those forces and the cognitive ability to do the hard, slow work of updating our beliefs when the evidence demands it may itself be under assault from a virus we still aren't taking as seriously as evidence warrants. And if that's true, then the parallel to Semmelweis isn't just a useful historical analogy, it's recursive.
Leslie Poston:We're watching people reject evidence about a thing that's degrading their ability to evaluate evidence. Semmelweis would recognize the familiarity of it immediately. Thanks for listening to PsyberSpace. I'm your host, Leslie Poston, signing off. Until next time, stay curious, and don't forget to subscribe so you never miss an episode.
Creators and Guests
