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Nature: "Hello? Yeah, I'm on the train..." Hearts sink throughout the carriage as fellow passengers try to ignore the ensuing conversation. But what exactly is it that makes overhearing others' mobile phone conversations so annoying? Helped by a group of unwitting passengers, a team of British psychologists think they have the answer.

You might imagine that it's simply a question of being riled by loud voices spouting inane drivel about people we've never heard of. But that's not the whole story, say Andrew Monk and his colleagues at the University of York, UK.

We also feel an innate need to listen when we can only hear one side of a conversation, the researchers say. Even if it's no louder than a regular two-way exchange, the fact that we can only hear half means that we instinctively tune in, almost as if we're expecting to join in to complete the conversation.

If this idea is correct, the researchers reason, then mobile phone chatter should be no more annoying than overheard conversations where both people are present but only one voice is audible. When Monk and his team tested their theory on railway passengers in Britain, that's exactly what they found.

Party time

They asked student volunteers to hold scripted conversations, either face to face or on a mobile, within earshot of an unsuspecting passenger. The content was typically banal: they enthused about a forthcoming holiday and a friend's party.

Afterwards, a researcher approached the passenger and asked them how annoying and intrusive they had found the conversation. As the team reports in the journal Behaviour and Information Technology1, passengers rated two-sided exchanges as less noticeable than both kinds of single-sided conversation.

The team staged the one-sided face-to-face conversations by ensuring that both volunteers walked past the victim before sitting behind him or her, to establish that two people were present. "On a train there's a lot of background noise - it's not unusual that one person's voice would be obscured," says Monk. "Nobody thought it was unnatural or staged."

Monk argues that the study shows why mobile-phone babble is intrinsically annoying. In Britain, some trains now have a carriage set aside as a phone-free zone. "We never had [designated] quiet carriages before mobile phones," he says.

Cell yell

The "need to listen" effect probably works alongside other factors that make cellphones annoying, such as loud speech. In a previous study, Monk's team found that bellowing voices are indeed more irritating than softly spoken ones2.

This "cell yell" may be due to the fact that phone users don't get any feedback to tell them their voice is too loud, says Lee Humphreys of the University of Pennsylvania in Philadelphia, who has carried out a survey of attitudes to mobile phone use.

Another factor that makes mobiles annoying is the content of conversations, Humphreys says. "People are fine if there seems to be a purpose to the conversation, perhaps to tell someone you're running late," she says. "Calling for the sake of it is much less acceptable."

Participants in her study also complained of being forced to listen to intimate details of phone users' lives. These included arguments, gory details of relationship break-ups, and even "intimate gynaecological information".

Mobiles are still a relatively new invention and it will take time for people to develop an agreed etiquette, says Monk. As ever with new technologies, the Japanese are leading the way. Polite mobile phone users in Japan tend to turn away and shield their conversations from others.

If that fails to catch on, there's always the less genteel approach, Monk says. Many theatres and train companies are considering investing in jamming devices that will thwart phone pests before they've even had a chance to assault us with their oh-so-catchy ringtone.

Michael Hopkin, 2004-10-05