Most of us are defensive about our driving abilities. We might be whizzing down the fast lane of the M5, squinting over the dashboard while burrowing in the glove compartment for a two-month-old packet of Werther's Originals, but we'd still claim that we're more responsible than some foolhardy chap we saw on a recent episode of 'Police Camera Action'. So if someone suggests that we lose concentration while using a hands-free mobile device, we'd say that it's no worse than other in-car distractions – arguing kids, errant satnavs, or hilariously bad dialogue on Radio 4's afternoon play.

But in the current 'Journal of Experimental Psychology', researchers at the University of Utah suggest that drivers are more likely to become distracted while talking on a hands-free set than while having a conversation with a passenger. One of the reasons they suggest is the associated benefit of having a second pair of eyes on the road; if you find yourself bewildered at Spaghetti Junction, it's likely that your passenger will a) shut up about their gallstones and b) help you navigate. You're not likely to get such consideration from someone at the other end of a phone line.

Of course, these things are hard to measure. The psychologists tasked with providing scintillating in-car conversation during the experiment might have been deeply tedious, thus freeing up the mind of the driver to concentrate on keeping the vehicle from ploughing into the central reservation. But there does seem to be something strangely stressful about using a hands-free phone while driving; I certainly feel more invested in the conversation than I would be if it were with a passenger.

Californian state law has already recognised this by banning the use of hands-free mobiles by under-18s. The mobile industry in the UK would no doubt defend to the hilt the safety of hands-free sets, while British motorists (particularly the ones who flout the existing law by jabbering away on normal mobiles while driving) would splutter in righteous indignation about the "nanny state" if such a law were ever introduced here. But it's a tricky one: do you legislate against people increasing their risk of having an accident, while knowing that there are greater risks associated with driving with acute hay fever symptoms; or while daydreaming about a colossal lottery win, which you can never legislate against?

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