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By Root 974 0

the post-hoc fallacy can generate accidental influentials, consider the following example from a real epidemic: the SARS epidemic that exploded in Hong Kong in early 2003. One of the most striking findings of the subsequent investigation was that a single patient, a young man who had traveled to Hong Kong by train from mainland China, and had been admitted to the Prince of Wales Hospital, had directly infected fifty others, leading eventually to 156 cases in the hospital alone. Subsequently the Prince of Wales outbreak led to a second major outbreak in Hong Kong, which in turn led to the epidemic’s spread to Canada and other countries. Based on examples like the SARS epidemic, a growing number of epidemiologists have become convinced that the ultimate seriousness of the epidemic depends disproportionately on the activities of superspreaders—individuals like Gaëtan Dugas and the Prince of Wales patient who single-handedly infect many others.7

But how special are these people really? A closer look at the SARS case reveals that the real source of the problem was a misdiagnosis of pneumonia when the patient checked into the hospital. Instead of being isolated—the standard procedure for a patient infected with an unknown respiratory virus—the misdiagnosed SARS victim was placed in an open ward with poor air circulation. Even worse, because the diagnosis was pneumonia, a bronchial ventilator was placed into his lungs, which then proceeded to spew vast numbers of viral particles into the air around him. The conditions in the crowded ward resulted in a number of medical workers as well as other patients becoming infected. The event was important in spreading the disease—at least locally. But what was important about it was not the patient himself so much as the particular details of how he was treated. Prior to that, nothing you could have known about the patient would have led you to suspect that there was anything special about him, because there was nothing special about him.

Even after the Prince of Wales outbreak, it would have been a mistake to focus on superspreading individuals rather than the circumstances that led to the virus being spread. The next major SARS outbreak, for example, took place shortly afterward in a Hong Kong apartment building, the Amoy Gardens. This time the responsible person, who had become infected at the hospital while being treated for renal failure, also had a bad case of diarrhea. Unfortunately, the building’s plumbing system was also poorly maintained, and the infection spread to three hundred other individuals in the building via a leaking drain, where none of these victims were even in the same room. Whatever lessons one might have inferred about superspreaders by studying the particular characteristics of the patient in the Prince of Wales Hospital, therefore, would have been next to useless in the Amoy Gardens. In both cases, the so-called superspreaders were simply accidental by-products of other, more complicated circumstances.

We’ll never know what would have happened at Lexington on July 17, 1775, had Paul Revere instead ridden William Dawes’s midnight ride and Dawes ridden Revere’s. But it’s entirely possible that it would have worked out the same way, with the exception that it would have been William Dawes’s name that was passed down in history, not Paul Revere’s. Just as the outbreaks at the Prince of Wales Hospital and the Amoy Gardens happened for a complex combination of reasons, so too the victory at Lexington depended on the decisions and interactions of thousands of people, not to mention other accidents of fate. In other words, although it is tempting to attribute the outcome to a single special person, we should remember that the temptation arises simply because this is how we’d like the world to work, not because that is how it actually works. In this example, as in many others, common sense and history conspire to generate the illusion of cause and effect where none exists. On the one hand, common sense excels in generating plausible causes, whether special people, or