Why are other people’s phone conversations so annoying? A new study suggests that hearing only half a conversation is more distracting than other kinds of conversations because we're missing the other side of the story and so can't predict the flow of the conversation. This finding suggests that driving a car might be impaired not only by the driver talking on the phone, but also by passengers talking on their phones.

It also tells us something about the way we listen to people talking — we’re actively predicting what the person is going to say next. This helps explain something I’ve always wondered about. Listen to people talking in a language you don’t know and you’re often amazed how fast they talk. See an audio recording of the soundwaves, and you’ll wonder how people know when one word starts and another begins. Understanding what people are saying is not as easy as we believe it is — it takes a lot of experience. An important part of that experience, it seems, is learning the patterns of people’s speech, so we can predict what’s going to come next.

The study showed that people overhearing cell phone conversations did more poorly on everyday tasks that demanded attention, than when overhearing both sides of a cell phone conversation, which resulted in no decreased performance. By controlling for other acoustic factors, the researchers demonstrated that it was the unpredictable information content of the half-heard conversation that was so distracting.