n the storm of disinformation since the emergence of Covid-19, the assertion that the virus is human-created has lingered on the fringes. This outlandish conjecture, once confined to conspiracy theorists, has undergone a renaissance after Joe Biden’s insistence that scientists should investigate the possible lab origins of Covid. From Vanity Fair to the Washington Post, the theory has been given a veneer of respectability. But there is an essential caveat that has been overlooked – that two different hypotheses are possible does not make them equally likely. Occam’s razor is a general rule of thumb, an injunction to “keep it simple”; when confronted with competing explanations for events, it is usually sensible to adopt the interpretation that pivots on the smallest number of supplementary assertions and assumptions. This is a principle readily applied to the competing narratives over the origins of Covid-19. Consider hypothesis one: Covid arose naturally. Supporting this is a history of the sudden emergence of devastating diseases; in fact, we have a grim abundance of prior examples. In the 1300s, the Black Death wiped out half of Europe’s population, while the 1918 flu pandemic killed tens of millions. Nor has modernity rendered us any less susceptible to the terrors of the microbiological; HIV, swine flu and Ebola are just a sampling of pathogens from the last 50 years. Aside from Covid, there have been at least two other coronaviruses (Sars and Mers) in the last 20 years. The incontrovertible conclusion is these pandemics arise frequently, without human intervention. Alternatively, there is hypothesis two: a lab leak. For this to be viable, we are obliged to add additional assumptions. We’d need to accept that the virus was engineered and subsequently released by accident or design. More damning for this narrative are the implicit temporal conditions it imposes: Wuhan, a city with a population of more than 11 million, with thriving wet markets, has millions of human-animal interactions each day, occasions when a virus could jump to humans. But the city has only a single virology lab where, accidentally or by design, everything would have to go wrong at once to yield the same result. The failings of the lab-leak idea are many because it requires a host of unlikely caveats to explain the observed data. By stark contrast, the natural origins hypothesis explains the same observations far more parsimoniously. If there were strong evidence for an additional assumption or supporting caveat, it should, of course, be accepted. Proponents of the lab-leak narrative insist there is: an explosive report in the Daily Mail, for example, carried claims from two scientists that Sars-Cov-2 was artificially created. One author even stated that “the laws of physics mean you cannot have four positively charged amino acids in a row. The only way you can get this is if you artificially manufacture it.” This astounding claim, however, has been utterly skewered. The biologist Michael Eisen dismissed it as “unbelievable bullshit”, noting that far from being unusual, “33% of human proteins have four consecutive positive-charged amino acids”. Similar dramatic claims by fringe experts that the virus might be artificially enhanced have been spread, but their inevitable refutations generate less excitement. There is simply no reputable evidence that the virus has been manipulated in any way. Nor has the existing evidence base changed – the World Health Organization’s Dr Mike Ryan lamented recently that “we have seen more and more discourse in the media, with terribly little actual news, or evidence, or new material”. A far less conspiratorial version of the conjecture also exists, which argues that the virus was discovered by researchers in the wild and inadvertently unleashed through accident or ineptitude. While this theory is less paranoid, it is plagued by the same deficits. To have “escaped”, the virus would have had to exist in nature to be sampled. No matter how massive the Wuhan Institute’s repository of bat viruses may be, the staggering number of viruses in nature would exceed any library by orders of magnitude. Nor is the current uncertainty over the animal genesis of Covid particularly suspicious; while Ebola was first recorded in 1976, we do not know how it emerged. Tracing the origin of a pathogen is laborious – it took 14 years for conclusive evidence that Sars arose from a virus transmitted from bats to civets to humans. None of this adds support to lab-leak narratives. The fixation on the origin of Covid is a distraction. It does not advance our understanding, nor address how we ought to proceed. While China may have questions to answer on its lack of transparency, fostering conspiracy theories is not conducive to overcoming the pandemic, nor to maintaining a spirit of collaboration. Throughout history, there has been an odious tendency to falsely attribute blame for pandemics, from assertions that Jews poisoned wells in the middle ages to decrying homosexuals for the rise of Aids. This has never been edifying or justified and we should strive to avoid it now. Lab-leak narratives risk emboldening conspiracy theorists. While hypothetically possible, they are not likely, nor corroborated by evidence, and obsessing on them is profoundly misguided. Carl Sagan’s dictum that “extraordinary claims require extraordinary evidence” is a principle we forget to our cost. Dr David Robert Grimes, a physicist and cancer researcher, is the author of The Irrational Ape: Why We Fall for Disinformation, Conspiracy Theory and Propaganda