We can walk and chew gum at the same time without bumping into utility poles. So what is the big deal about driving while talking on a cell phone?

Plenty. Investigators from the University of Sydney, the University of Western Australia and the Insurance Institute for Highway Safety in Arlington, Va., recently analyzed accident data and cell phone records of 744 drivers in Perth. They concluded that yakking drivers are four times more likely to crash their cars. And using a hands-free headset instead of a handheld phone made no difference at all. Several cities and states have forbidden the use of handheld, but not hands-free, phones in moving vehicles.

Independent studies confirm the risk and suggest that it is engaging in conversation, not manipulating a phone, that is most distracting. That argument you are having with your girlfriend over your hands-free phone is tying up neurons that could be better used to keep your Subaru between the lane lines. Experiments by Johns Hopkins University psychologists Sarah Shomstein and Steven Yantis on 11 volunteers show that the brain can be intensely aware of what is coming through either the eyes or the ears but not both at the same time. The subjects watched a stream of characters on a computer screen while listening to a voice chant a series of letters and numbers. Cues instructed the subjects to switch their attention between vision and hearing. Using magnetic resonance imaging, Shomstein and Yantis found that certain brain regions were activated when the subjects consciously chose to see; these were muted when they chose to hear.

Although music from the car radio or a conversation with a passenger may also compete for a driver's attention, listening is far more passive. “You don’t have to put resources into it,” Shomstein explains. “And a person sitting with you is aware of the situation as well as you are.” The individual in your earphone cannot see the toll plaza ahead.