As human beings, we pride ourselves on being the only species that understands the concept of risk. Yet we have a confounding habit of worrying about mere possibilities while ignoring probabilities—of building barricades against perceived dangers while leaving ourselves exposed to real ones.

For example, we agonize over the avian flu, which [as of December 2006] had killed precisely no one in the U.S., but have to be cajoled into getting vaccinated for the common flu, which contributes to the deaths of 36,000 Americans each year. White-knuckle flyers routinely choose the car when traveling long distances, heedless of the fact that, at most, a few hundred people die in U.S. commercial airline crashes in a year, compared with 44,000 killed in motor-vehicle wrecks.

We wring our hands over the mad cow pathogen that might be (but almost certainly isn't) in our hamburger, yet worry far less about the cholesterol that contributes to the heart disease that kills 700,000 of us annually. Shoppers still look askance at a bag of spinach for fear of E. coli bacteria while filling their carts with fat-sodden French fries and salt-crusted nachos.

We put filters on faucets, install air ionizers in our homes, and lather ourselves with antibacterial soap. At the same time, 20 percent of all adults still smoke; nearly 20 percent of drivers and more than 30 percent of backseat passengers don't use seatbelts; and two-thirds of us are overweight or obese.

In short, shadowed by peril as we are, you would think we'd get pretty good at distinguishing the risks likeliest to do us in from the ones that are statistical long shots. But you would be wrong.