For more than a decade we've run a series of seminars for federal judges. We don't teach law but rather explore contentious scientific issues with policy implications. Several hundred federal judges have come to Montana to learn about biotechnology, endangered species, climate change, and energy policy, for example.

Whatever the topic, ethical concerns always emerge. We are, after all, dealing with judges and they must, at root, consider the ethical implications of their decisions. Applying the law is not merely mechanical, it's judgmental. That's why we have judges, not computers, to decide controversies.

The interesting and problematic issues usually don't involve law breaking. Rather, they are contests between competing goods. The most difficult involve weighing tradeoffs among them. The most complex and subtle involve tradeoffs between what is good for an individual (or small group) and what is socially desirable.

Some individual decisions and actions affect people beyond those immediately involved. This "external" effect can be positive or negative. For example, an individual's choice to educate herself benefits not only her (e.g. through higher wages), but society at large by improving the quality of the workforce. Cigarette smoking has obvious negative "externalities" for nearby nonsmokers. The "Lug Nut Rule," named for the nuts holding the wheels to a vehicle, illustrates another example.

On some dimensions the U.S. would benefit if we used and imported less oil. Thus some favor policies forcing vehicles to have better fuel economy. Hence, in 1975 we passed CAFE.

CAFE is the acronym for Corporate Average Fuel Economy. The goal was to save oil by requiring automobile companies to meet fuel mileage standards averaged over the fleet of cars sold. "Light-duty trucks," and this includes SUVs, were held to less stringent requirements. This regulation had unintended consequences.

Here's why.

1) The laws of physics involving inertia, rolling resistance, and wind resistance dictate that it's easier to get high mileage from small cars than from large ones.

2) But most folks have strong reasons to prefer larger vehicles, namely safety, carrying capacity, and comfort.

3) Among their other disadvantages, small cars (those weighing under 3,200 lbs. and with only four lug nuts) cost lives. Larger vehicles have more and larger lug nuts, e.g., a 1/2-ton Chevy pickup has six lug nuts, a 3/4-ton has eight, and a 2-ton or larger has ten or more. Hence the number of lug nuts is a good proxy for size and safety. Thus the Lug Nut Rule, i.e. the more lug nuts, the safer. However handsome, dependable, and rugged, few people would want to drive a double-framed Autocar, International, Kenworth, Mac, or Peterbilt.

A National Academy of Sciences report released this year confirms the relative safety of larger vehicles. It concludes that "the downweighting and downsizing that occurred in the late 1970s and early 1980s, some of which was due to CAFE standards, probably resulted in an additional 1,300 to 2,600 traffic fatalities in 1993."

4) During the high oil prices of the 1970s, automotive companies had strong incentives to increase fuel efficiency. But mileage driven goes up with increased efficiency; folks drive more when it's less costly to do so.

5) Since "light trucks" have a lower CAFE requirement, companies made these trucks ever more car-like. We now have Lincoln and Caddy "trucks." These are actually Expeditions and Suburbans with Lincoln and Cadillac marques. Last month American auto companies for the first time sold more trucks than cars.

Economies are like ecosystems; elements within them evolve. Folks, both buyers and producers, adjust to changing circumstances.

Many Greens don't like this and throw hissy-fits. Given our revealed preferences, this implies wanting us to trade blood for oil. From this Green perspective, other people want the "wrong things."

In 1996 the EPA studied the costs and benefits of all air pollution controls imposed between 1970 and 1990. It found that controls on all automobile-related pollutants contributed less than 1 percent of the benefits from the Clean Air Act.

When we pick up judges at the airport for seminars on climate change and energy conservation, what vehicle do we use? A Suburban or Excursion, of course. Why? We want to increase the odds of bringing them back alive. The Lug Nut Rule trumps CAFE sentiments.