INTRODUCTION WHEN YOU TAKE RISKS, YOU ARE REMINDED IN THE MOST insistent manner that you have a body. For risk by its very nature threatens to hurt you. A driver speeding along a winding road, a surfer riding a monster wave as it crests over a coral reef, a mountain climber continuing his ascent despite an approaching blizzard, a soldier sprinting across noman''s land--each of these people faces a high chance of injury, even death. And that very possibility sharpens the mind and calls forth an overwhelming biological reaction known as the "fight-or-flight" response. In fact, so sensitive is your body to the taking of risk that you can be caught up in this visceral turmoil when death poses no immediate threat. Anyone who plays a sport or watches from the stands knows that even when it is "just a game," risk engages our entire being. Winston Churchill, a hardened campaigner from the most deadly wars, recognized this power of nonlethal risk to grip us, body and mind. When writing of his early years, he tells of a regimental polo match played in southern India that went to a tie-break in the final chukka: "Rarely have I seen such strained faces on both sides," he recalls.

"You would not have thought it was a game at all, but a matter of life and death. Far graver crises cause less keen emotion." Similar strong emotions and biological reactions can be triggered by another form of nonlethal risk--financial risk taking. With the exception of the occasional broker suicide (and these may be more myth than reality), professional traders, asset managers and individuals investing from home rarely face death in their dealings. But the bets they place can threaten their job, house, marriage, reputation and social class. In this way money holds a special significance in our lives. It acts as a powerful token distilling many of the threats and opportunities we have faced over eons of evolutionary time, so making and losing it can activate an ancient and powerful physiological response. In one important respect, financial risk carries even graver consequences than brief physical risk.

A change in income or social rank tends to linger, so when we take risks in the financial markets we carry with us for months, even years after our bets have settled, an inner biological storm. We are not built to handle such long-term disturbances to our biochemistry. Our defense reactions were designed to switch on in an emergency and then switch off after a matter of minutes or hours, a few days at the most. But an above-average win or loss in the markets, or an ongoing series of wins or losses, can change us, Jekyll-and-Hyde-like, beyond all recognition. On a winning streak we can become euphoric, and our appetite for risk expands so much that we turn manic, foolhardy and puffed up with self-importance. On a losing streak we struggle with fear, reliving the bad moments over and over, so that stress hormones linger in our brains, promoting a pathological risk aversion, even depression, and circulate in our blood, contributing to recurrent viral infections, high blood pressure, abdominal fat build-up and gastric ulcers. Financial risk taking is as much a biological activity, with as many medical consequences, as facing down a grizzly bear. This statement about biology and the financial markets may sound strange to ears accustomed to the teachings of economics.

Economists tend to view the assessment of financial risk as a purely intellectual affair-- requiring the calculation of asset returns, probabilities and the optimal allocation of capital--carried on for the most part rationally. But to this bloodless account of decision making I want to add some guts. For recent advances in neuroscience and physiology have shown that when we take risks, including financial risk, we do a lot more than just think about it. We prepare for it physically. Our bodies, expecting action, switch on an emergency network of physiological circuitry, and the resulting surge in electrical and chemical activity feeds back on the brain, affecting the way it thinks. In this way body and brain twine as a single entity, united in the face of challenge. Normally this fusion of body and brain provides us with the fast reactions and gut feelings we need for successful risk taking. But under some circumstances the chemical surges can overwhelm us; and when this happens to traders and investors they come to suffer an irrational exuberance or pessimism that can destabilize the financial markets and wreak havoc on the wider economy.

To give you a mere inkling of how this physiology works, I am going to take you onto the trading floor of a Wall Street investment bank. Here we will observe a high-stakes world where young bankers can step up or down a full social class in the space of a single bonus season, one year buying a beach house in the Hamptons, the next pulling their kids out of private school. So consider if you will the following scenario, in which an unanticipated and important piece of news impacts an unsuspecting trading floor. INCOMING! It has been said of war that it consists of long stretches of boredom punctuated by brief periods of terror, and much the same can be said of trading. There are long stretches of time when little more than a trickle of business flows in through the sales desks, perhaps just enough to keep the restless traders occupied and to pay the bills. With no news of any importance coming across the wire, the market slows, the inertia feeding on itself until price movement grinds to a halt. Then, people on a trading fl oor disappear into their private lives: salespeople chat aimlessly with clients who have become friends; traders use the lull to pay bills, plan their next ski trip or talk to headhunters, curious to know their value on the open market. Two traders, Logan, who trades mortgage-backed bonds, and Scott, who works down the aisle on the arbitrage desk, toss a tennis ball back and forth, taking care not to hit any salespeople.

This afternoon the Federal Reserve is holding a meeting of its Board of Governors, and normally these events are accompanied by market turbulence. It is at these meetings that the Fed decides whether to raise or lower interest rates, and should it do so it announces its decision at 2:15 p.m. Even though the economy has been growing at a healthy clip and the stock market has been unseasonably, even irrationally, strong, the Fed has dropped few hints of an increase. So today it is widely expected to leave rates unchanged, and by late morning most people across the trading fl oor haven''t a worry on their minds, and think of little else but whether to order sushi or pasta for lunch. But just before noon there comes the merest breath of change, rippling the surface of prices. Most people on the floor do not consciously notice it, but the slight tremor registers nonetheless. Maybe their breathing quickens, maybe muscles tense just a bit, maybe arterial blood pressure increases ever so slightly.

And the sound of the floor shifts, from the quiet buzz of desultory conversation to a mildly excited chatter. A trading floor acts as a large parabolic reflector, and through the bodies of its thousand odd traders and salespeople it gathers information from faraway places and registers early signals from events that have yet to happen. The head of the trading floor looks up from his papers and steps out of his office, surveying the floor like a hunting dog sniffing the air. An experienced manager can sense a change in the market, tell how the floor is doing, just from the sight and sound of it. Logan stops in midthrow and looks over his shoulder at the screens. Scott has already wheeled his chair back to his desk. Their monitors display thousands of prices and flowing news feeds, blinking and disappearing. To outsiders the vast matrix of numbers seems chaotic, overwhelming, and finding the significant bit of information in the mess of prices and irrelevant news items seems as impossible as picking out a single star in the Milky Way.

But a good trader can do just that. Call it a hunch, call it gut feeling, call it tradecraft, but this morning Scott and Logan have sensed a kaleidoscopic shift in price patterns well before they can say why. One of the brain regions responsible for this early warning system is the locus ceruleus (pronounced ser- u -leus), so called because its cells are cerulean, or deep blue. Situated in the brain stem, the most primitive part of the brain, sitting atop the spine, the locus ceruleus responds to novelty and promotes a state of arousal. When a correlation between events breaks down or a new pattern emerges, when something is just not right, this primitive part of the brain registers the change long before conscious awareness. By doing so it places the brain on high alert, galvanizing us into a state of heightened vigilance, and lowering our sensory thresholds so that we hear the faintest sound, notice the slightest movement. Athletes experiencing this effect have said that when caught up in the fl ow of a game they can pick out every voice in the stadium, see every blade of grass. And today when the stable correlations between asset prices broke down the locus ceruleus tripped an alarm, causing Scott and Logan to orient to the disturbing information.

Moments after Scott and Logan have preconsciously registered the change, they learn that one or two people on the Street have heard, or suspected, that the Fed will raise interest rates this afternoon. Such a decision announced to an unprepared financial community would send a tidal wave of volatility through the markets. As the news and its implications sink in, Wall Street, only a short while ago looking forward to calling it an early day, roils with activity. At hastily org.