Complex systems have a number of other essential features that affect how they respond to stress and also whether we can predict their future behavior.

Sometimes, for instance, small changes in a complex system produce huge effects, while large changes make little difference at all.24 In other words, cause and effect aren't proportional to each other. Specialists call this nonlinear behavior, and we encounter it all the time in our daily lives—even in relatively simple systems. A warming of one degree in temperature in our kitchen's freezer may be imperceptible to touch, but it can thaw all our food. A light switch doesn't budge with a gentle push, but apply slightly more pressure and it suddenly flips from off to on.

In the case of a complex system, nonlinear behavior can happen as disturbances or changes in the system, each one relatively small by itself, accumulate. Outwardly, everything seems to be normal: the system doesn't generate any surprises. At some point, though, the behavior of the whole system suddenly shifts to a radically new mode. This kind of behavior is often called a threshold effect, because the shift occurs when a critical threshold—usually unseen and often unexpected—is crossed. (In our everyday conversation, when we say something was "the straw that broke the camel's back," we're saying it caused a threshold effect.)

Threshold effects can be good or bad for us, depending on the circumstances and one's point of view. The end of apartheid in South Africa and the collapse of the Grand Banks cod fishery are both great examples of threshold effects, but the former was a positive development for many people and the latter wasn't. The international economy often exhibits threshold effects. The 1997-98 Asian financial crisis was a sobering case. A devaluation of the Thai bhat, a minor currency, launched a financial crisis that ricocheted through the international economy for months, cost trillions of dollars in lost economic output, and threw tens of millions of people out of work. One day the Asian economy was booming; the next it was in a nosedive.

We often see beneficial threshold effects in the evolution of technologies: when just the right confluence of enabling factors occurs, technological progress surges. For instance, once lots of people were using the Internet and once an effective browser had been invented, the World Wide Web spread around the planet like wildfire—with the number of Web servers (the powerful computers that host Web pages on the Internet) soaring from a few thousand in the mid-1980s to over two million by 1994 and almost 400 million now.25

The behavior of a complex system with these features is highly contingent—how it behaves at any given time, and how it evolves over time, depends on a host of factors, large and small, knowable and unknowable. I've come to think of such systems as encountering many junctions as they move through time—just like the junction that Robert Frost's traveler encounters in his famous poem "The Road Not Taken."

Two roads diverged in a yellow wood, And sorry I could not travel both And be one traveler, long I stood And looked down one as far as I could To where it bent in the undergrowth

Frost's traveler, like all human beings, is a complex system. The route we take through the woods depends on the number of junctions we encounter, on the number of paths available at each junction, and on countless subtle, imponderable things that influence us to choose one path over another at each junction. We can't hope to forecast our ultimate route. And the same is true of any other complex system. The further we try to predict into the future, the more bewildering the task of predicting the system's route becomes.

Once a complex system goes down a particular path, it can't easily jump from one path to another or retrace its steps to try a different path. Frost seemed to understand this inescapable feature of our world. After having made a choice to follow one path, his traveler laments,

Oh, I kept the first for another day! Yet knowing how way leads on to way, I doubted if I should ever come back.

Specialists have a term to describe this characteristic of complex systems: path dependent. Where the system is at any particular time depends on the tortuous, circuitous route by which it got there—"how way leads on to way," as Frost marvelously puts it. A complex system's history turns out to be crucially important because it profoundly shapes what the system becomes, and it can't be rewritten or repealed. Frost finishes,

I shall be telling this with a sigh Somewhere ages and ages hence: Two roads diverged in a wood, and I—

I took the one less traveled by, And that has made all the difference.

Many people focus on the decision made in the poem's penultimate line, taking it as an affirmation of why it's important to be different from the crowd. But I believe Frost's real message is in the last line, and it's much more disturbing and, in the end, poignant. He is telling us that a choice that appears insignificant can "make all the difference," and that there may be no going back.

When small things can make a big difference, and when it's impossible to know which small things matter and which don't, predicting the

The traveler confronts path dependency.

future becomes formidably difficult. This is especially true of human affairs.26 Even more than the behavior of other complex systems, human social, economic, and political behavior is often extremely sensitive to serendipity, to fad and the whims of leadership, and to sudden technological, economic, political, and environmental developments. Also, we can't know exactly when or how any complex system that crucially affects our lives will cross a critical threshold and flip to a new mode of behavior. Thirty years ago, who anticipated the implosion of Soviet communism, the widespread adoption of personal computers, the emergence of AIDS, or the opening-up of a gaping hole in the stratospheric ozone layer over the Antarctic? Or, for that matter, on September 10, 2001, who among us predicted that terrorists would fly planes into the World Trade Center?

Try it. Try to come up with a plausible scenario for what the world will look like in, say, 2025, or even 2015. After even a moment's reflection, you'll realize that the range of possibilities is almost infinite and that given the blazing rate of change in today's world, there's something profoundly unknowable about the future, even a future that could arrive within a decade or two.27

Because we don't have anything firm to guide us, when we try to predict what our world will be like, we tend to think that things will continue the way they are going now. If a technology like the computer microchip has steadily improved in one direction—in the microchip's case, by doubling its power every eighteen months—we tend to assume that the trend will continue in the same direction in the future. We also fall back on our underlying personal temperaments. Our natural optimism or pessimism powerfully affects whether we believe technological, social, and environmental trends will bring us happiness or grief. And in our data-saturated lives, it's easy to find evidence that confirms our biases.

Few people actually recognize how bad we are at predicting the future. Recently, though, I discovered an exception in an obscure collection of essays written at the time of the 1893 Chicago World Exposition. Various famous Americans were asked to describe what American society would be like a century hence—in 1993. In today's light, many of their predictions seem downright bizarre, and almost all are infused with the exuberant American optimism of the late nineteenth century—only two decades before the twentieth century's calamities began to unfold. To be fair, though, envisioning life a hundred years in the future is astonishingly hard, as is thinking free of our culture's dominant sentiments.

But one comment especially caught my eye. In his short essay, "The Future Is a Fancyland Palace," James William Sullivan, a prominent newspaper editor and a follower of the American economist and reformer Henry George, offered more insight than all the book's other prognostications. And in his poetic yet blunt acknowledgment of our inability to see the future, Sullivan was far more sensible than most of our "experts" today:

I find that I am unable to prophesy. The future is a fancyland palace whose portals I cannot enter. Moving toward it from Here, I am charmed with its brilliant façade. What sculptured splendors—porticoes, pillars, statues, windows! What is within? As I advance, however, the airy structure recedes. I cannot push beyond its threshold; its doors never open; on their other side are silence and mystery.28

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