contributed significantly to a long recession running from 1987 until 1993. Possible causes Discussions of the causes of the Black Monday crash frequently focus on two theoretical models, which differ in whether they focus on variables that are exogenous or endogenous. The first framework searches for exogenous factors, such as significant news events, that affect investor perceptions and behavior. These events are taken as "triggers" of market behavior. The second, "cascade theory" or "market meltdown", attempts to identify endogenous internal market dynamics and interactions of systemic variables or trading strategies such that an order imbalance leads to a price change, this price change in turn leads to further order imbalance, which leads to further price changes, and so on in a spiralling cascade. It is possible that both could occur, if a trigger sets off a cascade. Market triggers The crisis affected markets around the world; however, no international news event or change in market fundamentals has been shown to have had a strong effect on investor behavior. Several exogenous events have been cited as potential triggers for the initial fall in stock prices: a general feeling that stocks were overvalued and were certain to undergo a correction, the decline of the dollar, persistent trade and budget deficits, a proposed tax change that would make corporate takeovers more costly, rising interest rates, and uncertainties regarding the Louvre Accord. Sources have questioned whether these news events led to the crash. Nobel-prize winning economist Robert J. Shiller surveyed 889 investors (605 individual investors and 284 institutional investors) immediately after the crash regarding several aspects of their experience at the time. Only three institutional investors and no individual investors reported a belief that the news regarding proposed tax legislation was a trigger for the crash. According to Shiller, the most common responses were related to a general mindset of investors at the time: a "gut feeling" of an impending crash, perhaps brought on by "too much indebtedness". Market meltdown De-linked markets and index arbitrage Under normal circumstances the stock market and those of its main derivatives–futures and options–are functionally a single market, given that the price of any particular stock is closely connected to the prices of its counterpart in both the futures and options market. Prices in the derivative markets are typically tightly connected to those of the underlying stock, though they differ somewhat (as for example, prices of futures are typically