Our new “value” market continues to produce swings that leave stockholders reeling. The NASDAQ soared 242 points or 7.9 percent Oct. 13, the second biggest percent change in the market’s history. It quickly lost most of that gain, only to rise another 247 points Oct. 19. At that time, many analysts expressed the hope that the market for technology stocks had finally “bottomed out.” But it wasn’t to be. Last Wednesday the NASDAQ dropped another 190.22 points, losing 5.56 percent of its value. True, recent price movements have been tied to earnings reports and international events in a way that they weren’t last winter. But stock prices have been moving with a violence unjustified, in orthodox “rationalist” accounts, by the data coming in.

Take the case of Nortel Networks, the Canadian telecom equipment maker whose disappointing earnings report precipitated the NASDAQ’s plunge last Wednesday. The news that Nortel’s earnings were $300 million short of expectations resulted in a $55 billion drop in the company’s market value. That doesn’t seem quite right, does it? And investors fled from other fiber-optics stocks as well, even though it was far from clear that they were experiencing the same problems as Nortel. It used to be that investors were happy to buy a stock with barely a glance at earnings reports, now they panic at the first sign of a shortfall.

So maybe the market’s dizzying movement should dispel the hope that we have returned to the safe harbor of rationality, basing our investment decisions on how a company is actually performing, rather than how popular it is with investors. But I would propose an alternate conclusion — that the market is just as rational as it was last year, or even 10 years ago. And that, in fact, rationality is a pretty useless way of talking about the stock market, one that imputes motive to a inanimate system, mischaracterizes the reasons why people buy stocks and misapprehends the real function of valuation techniques and the analysts who use them.

By way of analogy, I’ll offer a brief anecdote. I recall a Spanish professor from my freshman year of college, a man of luxuriously curling hair and left-wing politics, who took great pleasure in challenging my Enlightenment-era worldview. “Man,” he declared (in Spanish), “is the least rational of the animals. If an animal is hungry, it eats. When tired, it sleeps. Humans are the irrational ones. We are the ones who starve ourselves to protest injustice, who toss and turn in an agony of unrequited love.” I was dismayed. Wasn’t Man defined by his rationality? Wasn’t our ability to Reason and to apply Logic what set us apart from other animals? But I couldn’t quite disprove his formulations. In the end, I threw up my hands — perhaps the true nature of man was unknowable. Looking closer, however, I can see that our vague use of the word “rational” had obscured the more concrete questions we had raised. The same is true for the stock market debate.

Fundamentally, an investor can obtain profit from a stock in two ways: by receiving dividend payouts from the company or by earning capital gains through selling the stock after it has risen in price. In today’s market, the latter reason has come to dominate investor motivation. In order for a stock to rise in price, other people must buy it. It is therefore “rational” for an investor to try to predict the inclinations of the rest of the existing and potential investor community. The question of whether a company is, in some objective sense, “good” or strong or profitable is really irrelevant to the investor, except as it affects others investors’ attitudes toward its stock. If other investors want it, then it’s valuable.

True, a share of stock is by definition a percent ownership in a company. But this is a legal abstraction rarely relevant in the real world. (Bankruptcy is the principal exception.) It is possible for a stock price to move independently or even inversely to the performance of the company. There is no reason why a drop in quarterly earnings must lead to a decline in stock price, except that each stockholder fears that others, reacting to the information, will sell their stock and wishes to escape the resulting depreciation of his assets. If investors could be persuaded that others would not sell their stock, the price would not fall. The issue, essentially, is the actions of other investors, not the status of the company.

Despite this fact, most stock analysts devote less time to an attitudinal investigation of the investing community than to an analysis of the so-called fundamentals: price/earnings ratio, dividend levels, etc. It’s not hard to see why. “Fundamental” research yields a more concrete, quantitative result and lends the financial professions greater legitimacy. Furthermore, in times of relative market stability, research into the fundamentals will lead to the same conclusions. All the other analysts are looking at the same data and the investing public is relying on their advice.

But methods of valuing stock are only as effective as the number of investors who subscribe to them. Those who discarded their price-equity calculations were scolded by purists for not sticking by the tried and true markers of rationality. But why should they, when the only reason that the P/E ratio was helpful in predicting stock prices was that everyone else was using it as well? Similarly, if the P/E ratio has become less helpful, it’s not because of new technologies or changing economic dynamics, but simply because enough people were persuaded that the old valuation methods were no longer where it’s at.

How is this related to my former professor’s provocations? It suggests that the way we have been discussing the stock market and investor behavior is inaccurate. Markets are “rational” in the sense that investors’ behavior is directed toward their self-interest and they will generally make reasonable choices to promote that interest. The problem occurs when the word “rational” is used as a substitute for “efficient,” or to suggest that stock price should conform to other measurements of the value of a company. Often stock prices will reflect these external value measurements, but only when investors are collectively possessed by the belief that it should. The discarding of P/E ratio and other traditional valuation methods may not be a good thing. It may contribute to the market’s volatility, exposing the investor to greater risk and companies to greater instability. But the Internet stock boom did not make the market more or less “rational.”

If anything, last year’s boom — and this month’s roller coaster — have only exposed the extent to which market movement is, and has always been, based on perception, psychology and convention.