Bernanke, Ben

[I]t is reasonable to ask whether systemic risks can in fact be reliably identified in advance... To respond to this point, I will distinguish, as I have elsewhere, between triggers and vulnerabilities. The triggers of any crisis are the particular events that touch off the crisis--the proximate causes, if you will. For the 2007-09 crisis, a prominent trigger was the losses suffered by holders of subprime mortgages. In contrast, the vulnerabilities associated with a crisis are preexisting features of the financial system that amplify and propagate the initial shocks. Examples of vulnerabilities include high levels of leverage, maturity transformation, interconnectedness, and complexity, all of which have the potential to magnify shocks to the financial system. Absent vulnerabilities, triggers might produce sizable losses to certain firms, investors, or asset classes but would generally not lead to full-blown financial crises