Financial crises are rarely the result of a single causal factor or a single set of circumstances. In the lead-up to a crisis, economic, psychological, social, and other variables interact in ways that lead to vulnerabilities and increase the probability that a wrong turn of events will trigger a crisis in the financial system. Sometimes crises are triggered by recessions and related economic pressures, while at other times they are the causes of recessions. Crises sometimes involve fraud in a significant way, but most frequently and in very general terms, they are more likely to result from simple human errors compounded by well-intentioned but inadequate rules and organizations. During economic expansions, overconfidence bias expands and when reinforced by the social phenomena of herd behavior, the result can be too much risk, too much debt, and increasingly fragile economies.