"Cell yell" isn't the only thing that makes overhearing a mobile phone call so intrusive. According to University of York psychologist Andrew Monk, hearing only one side of the conversation is what really drives us to distraction.

Monk and colleagues found that British Railways passengers were only mildly disturbed to overhear two people chatting in the seat behind them. But when only one side of the conversation was audible (because the other party spoke very quietly), passengers rated it just as annoying as a cell phone call. The findings were published in Behaviour & Information Technology.

Hearing only one side of a conversation compels us to pay extra attention to it, Monk explains. The "need-to-listen" effect isn't due to morbid curiosity, however. He theorizes instead that our brains, expecting information to arrive from both sides of a conversation, have an innate tendency to try to fill in the blanks—whether we like it or not.

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