In some respects, the findings are curious, Emberson said, because when overhearing half a conversation you're actually hearing less sound than you'd hear while listening to a full conversation.
But much of how humans process language is based on the brain's ability to predict, or anticipate, what comes next in a sentence, Emberson said. The brain is attuned to patterns of speech, acoustics and grammar, so when someone says, "I like to eat", the brain is already figuring out that what comes next is probably going to be some type of food.
One-sided conversations make it more difficult for the brain to make these predictions, so listening is more distracting, she said.
"With half a conversation we get less information, so we have to listen more," Emberson said.
It's not just in speech that humans count on being able to predict what's going to happen next, said Gerry Altmann, a professor of psychology at the University of York in England.
"There is increasing research in a whole range of areas to do with human behavior that shows our ability to predict what's going to come next is fundamental to both our mental and physical functioning," Altmann said. "Much of the behavior we engage in is, in fact, very predictable."
Altman likened hearing half a phone conversation to the difference between walking on a smooth sidewalk to an uneven, rock-strewn path. One doesn't require much attention. The other, which is unpredictable, requires more attention. An overheard cell phone conversation is different in that much of the process is subconscious, he added.
"With half a phone conversation, you're only getting a snapshot, so you can't predict the future, which is what the brain has evolved to do," Altmann said.
And with some 285 million cell phone subscribers in the United States, it's an irritation that isn't likely to go away.
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