The Golden Rule of Scientific Skepticism
Imagine astronomers detect a mysterious gas on a distant planet that could be a sign of alien life. Or a research team claims to have achieved the impossible: "de-extincting" a long-lost species. In our information-saturated world, revolutionary claims emerge constantly. But which deserve our belief and which warrant skepticism?
This dilemma is addressed by one of science's most famous principles: "Extraordinary claims require extraordinary evidence." Popularized by the beloved astronomer Carl Sagan, this concept serves as a bedrock of scientific skepticism and critical thinking everywhere 1 . It represents the ideal balance between open-minded inquiry and rigorous scrutiny—between the courage to explore radical ideas and the wisdom to demand compelling proof.
But what exactly makes a claim "extraordinary"? What constitutes "extraordinary evidence"? And how does this principle apply to today's most controversial scientific frontiers? The answers take us on a journey through centuries of philosophical debate and right up to the cutting edge of modern discovery.
While Carl Sagan made the phrase famous in his 1979 book Broca's Brain and 1980 Cosmos series, the underlying concept predates him by centuries 1 2 . The Scottish philosopher David Hume articulated a similar principle in his 1748 essay "Of Miracles," writing that when facts "partake of the extraordinary and the marvellous," the evidence supporting them "admits of a diminution, greater or less, in proportion as the fact is more or less unusual" 1 .
Other thinkers expressed parallel ideas. French scholar Pierre-Simon Laplace wrote in 1814 that "the weight of evidence for an extraordinary claim must be proportioned to its strangeness" 1 . Thomas Jefferson, expressing skepticism about meteorites in an 1808 letter, noted that claims "bearing no analogy with the laws of nature as yet known to us" need "proofs proportioned to their difficulty" 1 .
The specific phrasing we know today emerged through the work of sociologist Marcello Truzzi, who used "Extraordinary claims require extraordinary proof" in his 1978 article for Zetetic Scholar 1 . Sagan, moving in similar skeptical circles as Truzzi in the 1970s, slightly reworded it to the now-famous version and popularized it to a global audience 2 .
This "Sagan Standard," as it's sometimes called, quickly became a fundamental principle of scientific skepticism 1 . It embodies the scientific spirit of taking nobody's word at face value—of requiring solid evidence before accepting remarkable assertions.
David Hume articulates the concept in "Of Miracles"
Pierre-Simon Laplace writes about evidence proportioned to strangeness
Marcello Truzzi uses "Extraordinary claims require extraordinary proof"
Carl Sagan popularizes the principle in his book and TV series
At first glance, identifying "extraordinary" claims might seem straightforward, but the concept contains subtle nuances. According to Mick West of Skeptical Inquirer, an extraordinary claim isn't merely something uncommon or rarely observed—it's one that "violates our understanding of the universe" 2 .
This distinction is crucial. Winning the lottery is statistically uncommon, but it doesn't contradict fundamental physical laws. Conversely, claims of psychic powers or propulsion systems that defy known physics would revolutionize our understanding of reality if proven true 2 .
Statistically, the "extraordinariness" of a claim relates to its prior probability—how likely it is to be true based on existing knowledge. Claims with extremely low prior probabilities require more robust evidence to become believable 1 .
This doesn't mean extraordinary claims should be automatically dismissed. Many established scientific truths—from quantum entanglement to continental drift—seemed extraordinary when first proposed. But they gained acceptance only after accumulating substantial evidence that overcame their initial improbability 7 .
David Hume illustrated this concept with the story of an Indian prince who had never experienced ice. When travelers described water becoming solid, the prince was rightly skeptical—the claim contradicted everything he knew about water's behavior. His visitors couldn't produce ice as evidence, so he remained unconvinced of what seemed like magic 2 .
While statistically improbable, winning the lottery doesn't violate our understanding of probability or physics. We accept such claims with ordinary evidence like a winning ticket or official confirmation.
A device that produces more energy than it consumes would violate the laws of thermodynamics. This claim would require extraordinary evidence to be considered credible.
In scientific practice, "extraordinary evidence" isn't a vague concept—it translates to specific, rigorous standards:
While many social sciences consider 95% confidence (p < 0.05) acceptable, extraordinary claims often require much higher standards. The "gold standard" in physics is 5-sigma certainty, meaning there's less than a 0.00006% probability the finding occurred by chance 4 .
High StandardExtraordinary findings must be reproducible by different research teams using different methods. The discovery of the Higgs boson involved multiple independent research groups who separately confirmed the finding before the Nobel Prize was awarded 4 .
VerificationClaims that challenge established understanding need converging evidence from different approaches. The case for cosmic acceleration earned a Nobel Prize because two rival teams using different datasets arrived at the same startling conclusion 4 .
CorroborationModern statistical approaches increasingly use Bayesian analysis, which calculates how much new evidence should update our prior beliefs 3 . This framework formalizes the intuition behind the Sagan Standard: the more unlikely a claim seems initially, the more compelling the evidence needed to make it credible.
Bayesian meta-analysis produces Bayes factors that quantify the strength of evidence for one hypothesis over another. For example, a Bayes factor of 3 indicates the evidence favors the alternative hypothesis over the null by 3:1 3 .
| Bayes Factor | Strength of Evidence |
|---|---|
| 1 to 3 | Anecdotal |
| 3 to 10 | Substantial |
| 10 to 30 | Strong |
| 30 to 100 | Very Strong |
| 100+ | Decisive |
Astrobiology provides perfect examples of the Sagan Standard at work. The detection of life beyond Earth would be among the most profound discoveries in human history—the epitome of an extraordinary claim 4 .
When scientists reported detecting dimethyl sulfide—a gas produced by marine plankton on Earth—in the atmosphere of exoplanet K2-18b, the story made global headlines. But the scientific community responded with caution, noting the detection hadn't met the standards for extraordinary evidence 4 9 . The signal was relatively weak (3-sigma), could potentially be explained by other molecules, and most importantly, hadn't been independently confirmed 4 .
History offers cautionary tales. In 1996, scientists presented evidence for fossilized bacteria in the Martian meteorite ALH 84001. While initially compelling, further analysis revealed non-biological explanations for the observed features 4 . The claim initially failed the repeatability test—a key component of extraordinary evidence.
The Sagan Standard is frequently invoked in evaluating paranormal claims like extrasensory perception (ESP) or remote viewing. Some parapsychologists argue they've produced statistical evidence that would be sufficient to prove ordinary claims, yet skeptics remain unconvinced 2 .
Psychologist Richard Wiseman famously stated about remote viewing: "I agree that by the standards of any other area of science that remote viewing is proven, but begs the question: Do we need higher standards of evidence when we study the paranormal? I think we do" 2 . The reasoning is straightforward: claims that would revolutionize our understanding of physics require correspondingly revolutionary evidence.
How should a thoughtful person apply the Sagan Standard today? The key is balancing open-mindedness with healthy skepticism:
Evaluate the claim's consistency with established knowledge—does it require overthrowing well-supported theories?
Assess whether the evidence is statistically significant, reproducible, and from multiple independent sources.
Consider whether researchers are making extraordinary claims based on ordinary evidence due to confirmation bias.
Remember that evidence matters more than eloquence—a claim being appealing or intuitively satisfying doesn't make it true.
"It seems to me what is called for is an exquisite balance between two conflicting needs: the most skeptical scrutiny of all hypotheses that are served up to us and at the same time a great openness to new ideas."
As we encounter increasingly remarkable scientific claims—from artificial intelligence achieving consciousness to revolutionary energy sources—the Sagan Standard remains an indispensable tool for navigating the flood of information. It encourages us to maintain the scientific spirit: wonder at the universe's possibilities, coupled with insistence on rigorous proof.
Despite its widespread appeal, the Sagan Standard faces significant criticisms. Most notably, what counts as "extraordinary" is somewhat subjective and depends on an individual's existing knowledge and beliefs 1 6 . To a quantum physicist, entanglement may seem ordinary; to most people, it seems miraculous.
This subjectivity creates potential problems. As scholar David Deming notes, the standard could potentially "suppress innovation and maintain orthodoxy" if applied too rigidly 1 . Many scientific breakthroughs initially seemed extraordinary but were later validated.
Some critics argue that ordinary evidence should suffice, even for extraordinary claims. Philosopher Theodore Schick contends that "extraordinary claims do not require extraordinary evidence" if they provide the most adequate explanation for available data 1 .
Similarly, some religious apologists question whether the standard is unfairly applied to miracles but not to other improbable events 1 6 . They note that people readily accept eyewitness testimony for winning the lottery—another extraordinary occurrence—while rejecting similar testimony for religious experiences 1 .