Whether the canopy of a rain forest or the trading floor of Wall Street, complex systems share certain characteristics. A small input to such a system can produce huge, often unanticipated changes — what scientists call “the amplifier effect.” A vaccine, for example, stimulates the immune system to become resistant to, say, measles or mumps. But administer too large a dose, and the patient dies. Meanwhile, causal relationships are often nonlinear, which means that traditional methods of generalizing through observation (such as trend analysis and sampling) are of little use. Some theorists of complexity would go so far as to say that complex systems are wholly nondeterministic, meaning that it is impossible to make predictions about their future behavior based on existing data.
When things go wrong in a complex system, the scale of disruption is nearly impossible to anticipate. There is no such thing as a typical or average forest fire, for example. To use the jargon of modern physics, a forest before a fire is in a state of “self-organized criticality”: it is teetering on the verge of a breakdown, but the size of the breakdown is unknown. Will there be a small fire or a huge one? It is very hard to say: a forest fire twice as large as last year’s is roughly four or six or eight times less likely to happen this year. This kind of pattern — known as a “power-law distribution” — is remarkably common in the natural world. It can be seen not just in forest fires but also in earthquakes and epidemics. Some researchers claim that conflicts follow a similar pattern, ranging from local skirmishes to full-scale world wars.
What matters most is that in such systems a relatively minor shock can cause a disproportionate — and sometimes fatal — disruption. As Taleb has argued, by 2007, the global economy had grown to resemble an over-optimized electrical grid. Defaults on subprime mortgages produced a relatively small surge in the United States that tipped the entire world economy into a financial blackout, which, for a moment, threatened to bring about a complete collapse of international trade. But blaming such a crash on a policy of deregulation under U.S. President Ronald Reagan is about as plausible as blaming World War I on the buildup of the German navy under Admiral Alfred von Tirpitz.