Published online 13 June 2008 | Nature | doi:10.1038/news.2008.889

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Car adornments betray a territorial mindset.

A blitz of bumper stickers could mean an assault for more than just your eyes. Visions of America, LLC / Alamy

Bumper stickers such as “Make Love, Not War” and “More Trees, Less Bush” speak volumes about a vehicle's driver — but maybe not in the way they might hope. People who customize their cars with stickers and other adornments are more prone to road rage than other people, according to researchers in Colorado.

The number of road rage incidents — bouts of aggressive driving such as speeding or tailgating, or confrontations with other motorists — has risen dramatically in recent years. In 1995 the American Automobile Association found 12,000 injuries and 200 deaths were linked to US road rage. In 2008, the numbers are estimated to exceed 25,000 injuries and 370 deaths, and many more road rage incidents, especially those that do not lead to injury, go unrecorded.

Psychologist William Szlemko and his colleagues at Colorado State University in Fort Collins wondered whether increasingly crowded roads might be contributing to rising tempers. The volume of vehicles on US roads has gone up by 35% since 1987, whereas the road network has swelled by only 1%.

In humans, as in many other species, overcrowding leads to increased territorial aggression, and the team suspected that this was what was happening on the roads.

What are you driving at?

Szlemko and his colleagues quizzed hundreds of volunteers about their cars and driving habits. Participants were asked to describe the value and condition of their cars, as well as whether they had personalized them in any way.

The researchers recorded whether people had added seat covers, bumper stickers, special paint jobs, stereos and even plastic dashboard toys. They also asked questions about how the participants responded to specific driving situations.

To keep the participants from realizing that the team was collecting information about aggressive driving, questions such as “If someone is driving slow in the fast lane, how angry does this make you?” were interspersed with decoy questions such as “What kind of music do you listen to in the car?”. Szlemko's team used a pre-existing scale called “Use of vehicle to express anger” to diagnose the presence of road rage in their participants.

People who had a larger number of personalized items on or in their car were 16% more likely to engage in road rage, the researchers report in the journal <i>Applied Social Psychology</i>1.

Territorial disputes

“The number of territory markers predicted road rage better than vehicle value, condition or any of the things that we normally associate with aggressive driving,” say Szlemko. What's more, only the number of bumper stickers, and not their content, predicted road rage — so "Jesus saves" may be just as worrying to fellow drivers as "Don't mess with Texas".

Szlemko admits that he is not entirely surprised by the results. “We have to remember that humans are animals too," he says. "It's unrealistic to believe that we should not be territorial.”

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Precious little research has previously attempted to explore drivers' territorial feelings about their cars, says psychologist Graham Fraine at Queensland University's Transport Policy Office in Australia. “This work clearly demonstrates that people will actively defend a space or territory that they feel attached to and have personalized with markers,” Fraine says.

Szlemko suggests that this territoriality may encourage road rage because drivers are simultaneously in a private space (their car) and a public one (the road). “We think they are forgetting that the public road is not theirs, and are exhibiting territorial behaviour that normally would only be acceptable in personal space,” he says.

Although the finding will probably help psychologists to identify and potentially prevent road rage, the discovery may apply to other situations besides motoring. "I am curious to see if there is a correlation between marking other types of territories and other forms of aggressive behaviour," says psychologist William Wozniak of the University of Nebraska at Kearney.

Indeed, a brief glance around your office may reveal the most territorial individuals by the number of personalizing objects present on their desks.