How many projects and events occurred in the 20th century? Billions, trillions—who knows. But those eight alone impacted the world orders upon orders of magnitude more than others. The thing that makes tail events easy to underappreciate is how easy it is to underestimate how things compound. How, for example, 9/11 prompted the Federal Reserve to cut interest rates, which helped drive the housing bubble, which led to the financial crisis, which led to a poor jobs market, which led tens of millions to seek a college education, which led to $1.6 trillion in student loans with a 10.8% default rate. It’s not intuitive to link 19 hijackers to the current weight of student loans, but that’s what happens in a world driven by a few outlier tail events. The majority of what’s happening at any given moment in the global economy can be tied back to a handful of past events that were nearly impossible to predict. The most common plot of economic history is the role of surprises. The reason surprises occur is not because our models are wrong or our intelligence is low.