Today, let's look at the idea of risk homeostasis. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

All living is living at risk. Life without risk isn't life at all. Walking the dog or climbing the Matterhorn -- either one involves a level of risk. Either requires choices. Stepping into our bathtubs means balancing the small danger of breaking our necks against the great benefit of getting clean.

Writer Malcolm Gladwell talks about the way we fit risk into our lives. To begin with, he reminds us, we could never use any of our machinery if we demanded perfect safety of all components all the time. Most failures are highly unlikely, and, when they do occur, there's enough backup to protect us.

Now, Gladwell says, look at a serious breakdown -- say, the 1979 accident at the Three Mile Island nuclear power plant. It grew out of a cascade of five minor mishaps, no one of which represented serious danger. But acting together, a stuck valve, an obscured indicator, a broken gauge, and two other minor troubles brought the reactor to the brink of meltdown.

It's like the old idea that enough monkeys, typing at random on typewriters for a long enough time, would eventually produce the King James Bible. With so much technology in the world -- so many components -- every now and then a highly improbable group of nuisance flaws gives rise to a major disaster.

The 1986 Challenger explosion followed a different, but parallel, scenario. That explosion was caused by a single glaring O-ring failure. But here the chain of coincidence lay in the sequence of decisions. No one decision was rash, but, over time, a series of seemingly conservative decisions resulted in a launch that had to fail. O-rings on earlier flights had eroded without giving way. Something unsafe had begun to look safe.

So we reach the theory of risk homeostasis. It says we tend to accept a roughly constant level of risk. If life gets too safe, we begin taking chances. Pedestrians suffer more accidents in crosswalks than they do jaywalking. Jaywalkers know they're taking chances, so they're careful. Pedestrians in crosswalks take their safety for granted and fail to protect themselves.

When Sweden shifted from driving on the left side of the road, like the English, to driving on the right like the rest of Europe, they expected a rash of accidents. What happened was quite the opposite. The number of accidents fell dramatically as people became far more careful. One study shows that childproof lids on medicine bottles have led to more kids' getting into medicines. We became less careful about keeping medicines out of their hands.

So we incline to consume the safety of new devices and measures, instead of keeping it. In the face of that, the safest among us are probably those who accept some level of risk, then keep an alert respect for danger -- which is, after all, always present.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Gladwell, M., Blowup. The New Yorker, January 22, 1996, pp. 32- 36.

I am grateful to Art Pollet for providing this article and suggesting an episode based upon it.

For more on acceptable risk, see Episode 126.