The so-called American character usually turns out to be a chimera. Americans are said to be blunt to a fault, or desperate to please. They are too soft and complacent to stomach combat, or they are warmongers quick to sacrifice their children. They are merciless materialists, or else they are hopeless sentimentalists and irrational religious believers. One quality, however, has grown unmistakable in the eyes of the world, and now enjoys the consecration of statistical confirmation: Americans are fat. Between the early 1990s and the turn of the millennium, it seems that the percentage of Americans categorized as "obese," which is a large size indeed, climbed from below one in four to nearly one in three. The percentage considered "overweight," meantime, approached two in three, a larger majority than has voted for any presidential candidate since World War Two.

Explanations abound. Some say we human beings are programmed for life on the African savannah, where the food supply depended on whether some able young man had outrun or outwitted a springbok. Programmed to gorge when we have the opportunity and store up food energy for lean times, we are now facing the consequences of decades of plenty. Others accuse McDonald's and its ilk of foisting large portions of processed foods on consumers, and even literally addicting eaters to fatty, sugary meals. Still others contend that the trend against smoking has combined with the increased role of women in the workforce to add to Americans' weight: lack of nicotine slowed down the national metabolism, while the absence of homemakers drove husbands and children to quick, easy, and unhealthy processed food.

The common thread of these and other fashionable explanations is that they treat us as animals -- indeed, animals as Descartes imagined them, automata governed by blind instinct, no more self-conscious than the wind-up ducks that were the toys of kings. Such biological explanations of complex behavior are our age's version of the great reductionist theories of the past: that man was matter in motion, as for Hobbes, or a function of his place in the relations of production, as for some Marxists if not for Marx himself. The story of the new biological reductionism is very simple: people strive to pass on their genes, and hence to survive and flourish long enough to do so. To that end, we are equipped with a simple, invariant, and irrepressible set of appetites. Whatever setting we are dropped into, those instinctive appetites will assert themselves. In congenial settings, they help us to reproduce. In uncongenial circumstances, they lead us to fatten ourselves as if for slaughter.

It is strange that these theories should have such purchase in a culture obsessed with individuality, authenticity, and spirituality, all commitments that might seem at odds with a description of human life as the activity of mechanical ducks -- or, more aptly, hungry apes. Perhaps, paradoxically, such a potent theory produces in some Americans what a handgun gives others: a feeling of mastery over a complex and dangerous world. Never mind that the theory by its terms makes us the playthings of instinct -- as a handgun makes us more dangerous to ourselves as well as to others. The point is the illusory reassurance of possessing it. We will take that prize even at the cost of denying our own human complexity.

Be that as it may, people are not animals of the kind Descartes described. We remake our own instincts and desires by habit and culture, by reinterpreting and re-imagining ourselves. What satisfies us, what delights us, what we need, is always a matter of culture as well as nature. What, then, might Americans' weight problem say about the country's culture? And what, in this most political of years, might it say about American politics?

The signal fact about the last fifteen years of American culture has been the pursuit of sensation. Our video games come ever closer to being wraparound sensory experiences, immersing their players in battles and other adventures. Internet pornography has broken down many old inhibitions against this once-furtive industry, and has become the biggest commodity on the web. The growth area in American recreation is "extreme sports" -- from bungee jumping to stunt-snowboarding to super-fast, in-line skating down the dramatic hills of San Francisco.

From a biological perspective, these are tricks for inducing quick spikes of adrenaline in our systems, flooding us with the high -- and the mental concentration -- of fear, followed by the ecstasy of relief when we have come through safely. They fill our lives with little tonics that were once the fruits only of such supreme moments as successful hunts, fights to the death, and struggled-after sexual consummation. From a cultural and psychological perspective, they are opportunities for everyone to play the hero, to burst the bonds of convention and personal limitation into fantasies of extreme potency. And, at least on its face, the thrills are free: no one need be defeated for everyone to be a vanquisher. Whatever the ups and downs of reproductive competition, there is always pornography to go around.

Nor must the sensation always be heroism or sexual conquest. Writing nowadays -- once the model of solitary and reflective activity -- is usually punctuated by e-mail correspondence: news from the outside world; an event! In my generation as never before, students write and study in coffee shops, reporting that silence and solitude are too distracting: they need the small events of music, snatches of conversation, an attractive person entering the shop.

Eating, too, is an event, a sensation that helps to sprinkle our days with small antidotes to the creeping boredom of everyday life. It is something happening. The more accustomed we become to our doses of sensation, the more intolerable the boredom becomes, and the more it seems necessary to palliate ourselves with one chemical jump-start to another. A snack, a soda, a piece of cake at midnight, brings a blood-sugar spike; it is a well for restless people to do and feel something.

It might seem that the counter-trend to sensation-seeking would be the rise of evangelical Christianity, which has grown steadily more widespread in American life through at least the last three decades. George W. Bush, after all, emerged from pleasure-seeking years to an abstemious and purposeful life seemingly ordered by religious devotion. The contrast, though, may underestimate the continuity in the president's motivations from his addled youth to his sober middle years. Evangelical Christianity in the United States is much more sensation-seeking than meditative or reflective. The sensation it offers is the ecstasy of salvation. The narrative arc of such belief is youthful sin, dramatic salvation, and, often, "backsliding," a term indigenous to the American South, denoting a return to sinful ways before an even more dramatic and ecstatic return to salvation. Catholics and mainline Protestants are christened and confirmed, in a religious life founded on continuity; evangelicals are "born again" in the drama and excitement of discontinuity.

And what of politics? American sensation-seeking unsettlingly enacts Plato's famous description, in the Republic, of the character of democracy: men following whim and fashion with no clear plan of life, no principle, and no distinction between higher and lower pleasures. One day, Plato wrote, the democrat drinks late with his friends. The next day, he is determined to win back his health, and works out for hours at the gymnasium. This week he cares for nothing but politics, and is determined to master rhetoric. Next week has forgotten politics and cares only for soldiers. In a month it will be businessmen. In today's phrase, the democrat follows his bliss. The result is an undignified mess.

Is it true? Does freedom free us to debase ourselves? In answering the question, it may be helpful to stay with Plato for a moment. He was not attempting to write what we would now call political science, but instead was concerned with what we might call forms of character, or more pretentiously, states of the soul. By democracy he meant a condition in which all desires are equal, and the most powerful ones call the tune at any moment. (The analogy to the rule of the majority of equal citizens is still potent, but it is not necessarily illuminating.) This much is eerily prescient. American democracy takes place in a culture where desire is increasingly its own law, and no desire takes a back seat to any other. Contrary to the hopes of some American Romantics, such as Ralph Waldo Emerson and Walt Whitman, the result is not so often the free development of great souls as it is the free wandering of muddled ones.

But however compelling Plato's analogy may seem, the equality of desires is not the same as democracy, and democracy cannot be blamed for it. Given the opportunity, people have always designed heroic and erotic adventures for themselves, notably in the harems of sultans and the hunting expeditions of kings. What is new is that wealth and, especially, technology, have created nearly equal access to sensations that were once the preserve of the few, by substituting digital screens and refined sporting equipment for tigers and mistresses. It may be that the old American slogans -- "every man a king"; "an aristocracy of everyone" -- have come true, but that, alas, we had forgotten what a debauched thing the usual aristocracy is.

But that does not mean we should be complacent. Democracy introduces not the equality of desires, but a duty in ordinary people to make political judgments: to distinguish personal interest from collective interest, short-term from long-term interest, prudence from impulse, and, sometimes, impulse or interest from principle. It is easy to be too high-minded about this, of course; empirical research shows that American voters tend not to know all that much about the candidates and issues whose fates they decide. That said, however, they do know something, and they do think about what they know. If they did not, if they went for impulse and short-term interest every time or paid no attention at all to politics, the democracy that now limps along through intermittent disappointments and bursts of excellence would eventually run out of luck and fall into oligarchy or something worse.

Sensation-seeking is the enemy of political judgment. It encourages voters to think less and feel more. The equality of desires fosters the idea of the equality of spheres of activity: in work, in play, in romance, the question is, Does it feel good? Is it exciting? There may be something to be said for the power of this idea to free people from bad old conventions: that is certainly a plausible view when the feel-good standard opposes repressive marriages and dreary jobs. In politics, however, it is a mistake plain and simple. And in the shameless rhetoric of the Bush administration, it has become a formula for political manipulation.

Whatever the artistic failures and polemical exaggerations of Farenheit 9/11, Michael Moore was entirely right on one point: President Bush has mastered a crude orchestral composition of high-pitched fear on the one hand and booming patriotic fervor on the other. The formula has the familiar emotional arc of an action-thriller movie: terror, a fraught, heroic response, and a promise of ecstatic release at the end. I first understood that the Iraq invasion was probably inevitable when I realized, in a weekend of watching CNN in the summer of 2002, that the path to invasion was following this familiar narrative: a looming threat, a necessary and cathartic response, and in victory a reaffirmation of our goodness. The Republican National Convention was the administration's best shot at giving Bush's re-election the same aura of inevitability. Of course, sentimentalism is standard fare at political conventions, and patriotism is entirely proper. But in its exaggerations and outright lies, its conjuring of fear, and its bullying swagger, the convention -- like the entire Bush campaign -- was an insult to the intelligence and discernment of American voters. As I write, the polls suggest that it was also a success.

It is important not to make too much of one's historical moment. Historians report that Americans in other times have been almost as pervasively drunk as they are now pervasively overweight; before us, the English held that distinction, which they now look to be reclaiming in their pub districts. Walt Whitman lamented the universal selfishness and indifference to the public good that filled the country in the dark years after the American Civil War. We move from vice to vice, and our weak character has put democracy in danger before -- as it is in danger now. The present time is our particular version of a recurrent problem: democracy often fails to produce the qualities of character in its citizens that are necessary to preserve it. So far, we have muddled through more often than not; but that is not a cause for complacency.

We Americans owe it to the world to develop, alongside our robust capacity for terror and ecstasy and our hunger for intense experience, counterweight habits of reflection and prudence. Above all, we need to learn to distinguish between the judgment that politics requires and the sensation-seeking we allow ourselves in so many other areas. What we owe the world, though, is not a likely predictor of what we will do. For the time being, we will remain the sort of people we are, and the question will be whether the usual awkward combination of reason and passions will enable us to steer more or less straight, or at least straight enough to avoid the worst. If George W. Bush, a failed president and brilliant manipulator of sensation, can defeat John Kerry, that will be a very bad portent.