Black Monday 1987: A Fall Without a Cause

*In a single session, the American market fell by more than a fifth. Decades later, there is still no agreed explanation of what news, if any, prompted it.*

On Monday the nineteenth of October, 1987, the Dow Jones Industrial Average fell by more than twenty-two percent in a single trading session. It remains the largest one-day percentage decline in the history of the American market, exceeding anything recorded during 1929 or during any subsequent crisis. An investor who held a broadly representative American portfolio watched more than a fifth of its value disappear between the opening bell and the close of a single ordinary autumn day.

What makes the episode so unusual, and so instructive, is that no one has ever satisfactorily explained why. There was no bankruptcy of a major institution that morning. No war began. No government collapsed. No economic report of catastrophic significance was released. Investigators, economists, and historians have examined the episode for decades and have produced a range of contributing explanations, but nothing resembling a single triggering event that would account for a fall of that magnitude has ever been identified.

The explanations that have emerged concern the mechanics of the market rather than any development in the world outside it. A great deal of attention has focused on a practice then popular among institutions, which involved selling futures contracts automatically as prices fell in order to limit losses. The difficulty with such an arrangement is that it responds to a decline by generating further selling, which produces a further decline, which generates further selling. On that Monday, a substantial quantity of capital was operating under rules of this kind simultaneously, and the resulting cascade appears to have been self-reinforcing.

Other contributing factors have been proposed. The systems handling orders were overwhelmed by volume, so that participants could not reliably determine what prices actually prevailed, which introduced panic of its own. Market makers, faced with a flood of selling and no buyers, withdrew or widened their quotes. International markets had fallen before the American session opened, contributing to the atmosphere. None of these, individually or together, constitutes the kind of explanation that would make the fall seem reasonable.

The most important fact about the episode, however, is not the fall but what followed it. The American market recovered a substantial portion of the loss within days, and it recovered fully within roughly two years. The Dow, remarkably, finished the calendar year 1987 slightly higher than it had begun. An investor who had held their positions and done nothing whatever would have found, within a relatively short period, that the most dramatic single-day decline in market history had left almost no lasting mark on their wealth.

This produces a stark comparison with 1929, and the comparison is the reason both episodes belong in any serious education. The 1929 decline was less dramatic on any single day and took twenty-five years to recover. The 1987 decline was more dramatic than anything in 1929 and recovered within two. The severity of a single session, it turns out, carries almost no information about the severity of what follows, and an investor who reasons from the drama of a decline to the length of its consequences is reasoning from the wrong variable entirely.

The distinction that appears to matter is between a decline that reflects a genuine deterioration in the underlying economy and a decline that reflects the mechanics of the market itself. In 1929, the fall preceded and accompanied an economic catastrophe of the first order, and the recovery could not arrive until the economy recovered. In 1987, the American economy was in reasonable condition before the fall and remained so afterwards, and the market, having convulsed for reasons largely internal to itself, resumed its previous course.

This distinction is far easier to draw in retrospect than in the moment, and honesty requires acknowledging it. An investor watching a fifth of their capital vanish in a single session had no way of knowing whether they were living through 1929 or 1987. The information required to distinguish the two was not available. This is precisely why the response most likely to serve an investor well is one decided in advance, according to a framework, rather than one improvised on the day when the necessary information does not exist.

The episode also carries a lesson about automated arrangements that respond to price. Any mechanism that generates selling in response to falling prices will, if enough capital operates under the same mechanism, contribute to the very fall it was designed to protect against. The protection that appears sound when one participant employs it becomes destabilising when everyone does, and this property of markets, that a strategy can be undermined by its own popularity, recurs in many forms and is rarely anticipated.

One consequence of the episode deserves mention because it altered the structure of markets themselves. In its aftermath, mechanisms were introduced to halt trading temporarily when prices fall beyond defined thresholds within a session, on the reasoning that a pause allows participants to assess what is happening rather than continuing to sell into a cascade that has become self-sustaining. Whether such mechanisms genuinely help is debated, since a halt may simply postpone selling rather than prevent it, and it may add to anxiety by preventing participants from acting. What is not debatable is the recognition underlying them, which is that a market can, under certain conditions, produce movements that reflect nothing about the world and everything about its own machinery. An investor who understands that some portion of any given decline may be mechanical rather than informational will be considerably slower to conclude that a falling price is telling them something.

At VESTFY™ Black Monday is presented chiefly as an argument against reacting to price movements as though they were information. On that day, an enormous quantity of capital moved on the basis of price alone, without reference to any development in the world, and the movement proved almost entirely meaningless within two years. An investor who had responded to it would have been responding to nothing at all.