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The Environmental Consequences of Your Second Car

New research finds Californians who buy a fuel-efficient car often purchase a bigger, less environmentally friendly second vehicle.

Bad news for those of us who hoped the increasing popularity of hybrids and other high-mileage cars and trucks would be a game-changer in the fight against climate change. It seems we forgot about the licensing effect.

We're not talking about driver's licenses, or license plates. Rather, we're referring to the much-studied psychological phenomenon in which doing something good for society gives us license to commit a more self-centered act.

This reflexive response has been widely documented, including in a study that found people who purchased organic food were less likely to volunteer their time. It appears to be a common phenomenon: We earn brownie points (at least in our own minds), and then we spend them.

The latest example of this phenomenon has been identified by researchers from the University of California–Davis. They report that when households that purchase an environmentally friendly, energy-saving car later buy a second vehicle, it tends to be larger and less fuel-efficient.

This phenomenon, along with a tendency to take advantage of the lower operating costs of fuel-efficient cars by driving more, reduces the positive environmental impact of the purchase by as much as 60 percent, according to a research team led by UC–Davis economists James Archsmith and David Rapson.

A certificate of ownership does not automatically entitle you to a conscience-free SUV.

"Unintended consequences like this need to be taken into account when making policy," Rapson said in announcing the findings. "On average, fuel economy standards are putting more fuel-efficient cars in households. That can be good if it reduces gasoline use. But if it causes people to buy a bigger, less fuel-efficient second car to compensate, this unintended effect will erode the intended goals of the policy."

Their working paper, which has yet to be peer-reviewed, analyzed California Department of Motor Vehicles data on household vehicle purchases between 2001 and 2007. They estimate the tendency to buy a less-efficient second vehicle reduces the net fuel savings for an average household from around 68 gallons per year to between 24 and 27 gallons.

"We don't suggest getting rid of fuel economy standards and doing nothing else," Rapson said. "But our conservation policies need to be effective. A more promising solution is to put a price on carbon, such as what California is doing with cap and trade. It produces economic incentives that evidence shows will reduce carbon emissions."

It's important to reiterate that buying hybrids and other low-emissions vehicles does benefit the environment. Such vehicles burn less energy, even if they're only used for certain trips.

Just keep in mind that a certificate of ownership does not automatically entitle you to a conscience-free SUV.