Excerpt from
Uncommon Knowledge
It doesn't take a rocket scientist to understand that using a cellphone makes it harder to pay attention to other things going on around you. Thus, many states now restrict cellphone use while driving. Now, with a new study, it looks like they may have to consider curbing conversations by passengers, too. When people could hear one side of a phone conversation in the background, they performed worse on tasks that demanded attention. This did not happen to people overhearing the full dialogue, a monologue, or when the audio of the conversation was filtered so that its content could not be understood. All of this suggests that unpredictable speech is an extra distraction and may also explain why overhearing a cellphone conversation at a party or a movie can be more annoying than overhearing other conversations.
Emberson, L. et al., "Overheard Cell-Phone Conversations: When Less Speech Is More Distracting," Psychological Science (forthcoming).