After almost every great American tragedy, we go through a familiar cycle of self-examination and recrimination. There are presidential commissions and task forces, congressional hearings and investigations, press exposes and think tank reports. We have fact-finding and finger-pointing. The public excavation of prescient warnings, poor judgments and botched assignments leads sometimes to constructive conclusions, sometimes to extenuating circumstances and sometimes to a list of the guilty. We have already started this wrenching ritual with Hurricane Katrina.

But missing from these exercises is a candid acknowledgment of an underlying cause of many national tragedies: the human tendency not to contemplate the worst possibilities, which are usually hypothetical and uncertain. Most of us don't want to imagine future problems and horrors that could alter life as we know it. So we don't. We are, in any case, overwhelmed by predicted "crises" (terrorism, global warming, economic meltdowns, budget crackups and so forth), which are aggressively merchandised by ambitious politicians, energetic advocacy groups and an adversarial media. It is hard to know which of these menaces pose genuine threats and which are impostors. The unrelenting alarmism numbs us. Moreover, doing something about most of these purported dangers would be costly, inconvenient and contentious. To save our way of life, we need to alter our way of life.

The simplest and sometimes wisest response is to do nothing, which is what most of us do most of the time. The result is a sort of Catch-22 of national disasters: We cannot address serious national problems until they are conclusively shown to be serious, but the required proof is usually the very crisis that we are trying to avoid. In a democracy, it's necessary to mobilize public opinion to undertake unpleasant or expensive actions, but public opinion mobilizes only after the fact. In our world of crisis-mongering, we demand some means of distinguishing the real from the fraudulent. But the screening process is often an episode of national suffering.

We do not plan, even when the case for planning seems overwhelming. Examples abound. We know that over the next few decades the number of retirees will double and that the costs of federal retirement programs will explode, requiring huge tax increases (at least a third), unsustainably large budget deficits or deep (and undesirable) cuts in other government programs -- or some combination of all three. All of this has been evident for years: indeed, it is the subject of countless government reports. But successive presidents and Congresses have done little to change matters, the current stalemated Social Security "debate" being a case in point.

Or take oil. By the early 1970s, it was obvious that we should curb our use of oil, not because the world was running out of it (even now, that's not clear) but because its supply was inevitably insecure. Two-thirds of the world's known oil reserves lie in the unstable Middle East. Despite four oil "crises" since the early 1970s, we have yet to experience a truly catastrophic cutoff of global oil supplies, although that remains a possibility. Facing it, the country would be more secure with more efficient vehicles and a large Strategic Petroleum Reserve. But in the 1990s, fuel efficiency stagnated and the oil reserve actually declined slightly.

A final example: immigration. The Census Bureau reported last week that since 1989 about 70 percent of the increase in people below the government's official poverty line occurred among Hispanics. Over the same period, Hispanics accounted for more than half of the increase in people without health insurance. It seems incontestable that the uncontrolled immigration of poor Latinos increases poverty in the United States, even if many immigrants successfully assimilate (as they do). Yet, illegal immigration is rampant.

We have trouble taking costly and disruptive actions in the present to minimize more costly and more disruptive consequences in the future. To wit: Americans regard cheap gasoline as a quasi-constitutional right, not to be governed by the geography of oil. Similarly, they see Social Security and Medicare benefits as inviolate, not to be compromised by an aging population or longer life expectancies. We deal with inconvenient facts by ignoring them.

The post-Katrina investigations will reveal blunders and may improve our capacity to deal with future natural disasters or acts of terrorism. But we won't address the larger problem of public delusion, because it is so embedded in our democratic process. Not every national tragedy or crisis can be anticipated or avoided; but some can be defused or mitigated. Up to a point, you can blame politicians for not leading public opinion. But you can't blame them for not leading where the public steadfastly refuses to go.