Everything Is Obvious_ _Once You Know the Answer - Duncan J. Watts [53]
As the psychologist Robyn Dawes explains in his account of the accident, the investigation concluded that although no one of these factors—fatigue, communication mix-up, radio failure, weather, and stress—had caused the accident on its own, the combination of all five together had proven fatal. It seems like a pretty reasonable conclusion, and it’s consistent with the explanations we’re familiar with for plane crashes in general. But as Dawes also points out, these same five factors arise all the time, including many, many instances where the planes did not crash. So if instead of starting with the crash and working backward to identify its causes, we worked forward, counting all the times when we observed some combination of fatigue, communication mix-up, radio failure, weather, and stress, chances are that most of those events would not result in crashes either.4
The difference between these two ways of looking at the world is illustrated in the figure below. In the left-hand panel, we see the five risk factors identified by the Flight 2605 investigation and all the corresponding outcomes. One of those outcomes is indeed the crash, but there are many other noncrash outcomes as well. These factors, in other words, are “necessary but not sufficient” conditions: Without them, it’s extremely unlikely that we’d have a crash; but just because they’re present doesn’t mean that a crash will happen, or is even all that likely. Once we do see a crash, however, our view of the world shifts to the right-hand panel. Now all the “noncrashes” have disappeared, because we’re no longer trying to explain them—we’re only trying to account for the crash—and all the arrows from the factors to the noncrashes have disappeared as well. The result is that the very same set of factors that in the left-hand panel appeared do a poor job of predicting the crash now seems to do an excellent job.
By identifying necessary conditions, the investigations that follow plane crashes help to keep them rare—which is obviously a good thing—but the resulting temptation to treat them as sufficient conditions nevertheless plays havoc with our intuition for why crashes happen when they do. And much the same is true of other rare events, like school shootings, terrorist attacks, and stock market crashes. Most school shooters, for example, are teenage boys who have distant or strained relationships with their parents, have been exposed to violent TV and video games, are alienated from their peers, and have fantasized about taking revenge. But these same attributes describe literally thousands of teenage boys, almost all of whom do not go on to hurt anyone, ever.5 Likewise, the so-called systemic failure that almost allowed Umar Farouk Abdulmutallab, a twenty-three-year-old Nigerian, to bring down a Northwest Airlines flight landing in Detroit on Christmas Day 2009 comprised the sorts of errors and oversights that likely happen in the intelligence and homeland security agencies thousands of times every year—almost always with no adverse consequences. And for every day in which the stock market experiences a wild plunge, there are thousands of days in which roughly the same sorts of circumstances produce nothing remarkable at all.
IMAGINED CAUSES
Together, creeping determinism and sampling bias lead commonsense explanations to suffer from what is called the post-hoc fallacy. The fallacy is related to a fundamental requirement