Why do economies collapse into recession in ways that seem so difficult to predict? Why do financial markets collapse into free-fall with timing that's so loosely related to market valuations? Much of the reason is that complex systems usually aren't linear. In a linear system, a given change in some variable X always has the same expected effect on variable Y. In the economy and in the financial markets, the same event can have zero effect in some conditions and profound effect in another, often depending on much broader conditions that make the system vulnerable