Ever wonder why overhearing a cellphone conversation is so annoying? American researchers think they have found the answer.

Whether it is the office, on a train or in a car, only half of the conversation is overheard which drains more attention and concentration than when overhearing two people talking, according to scientists at Cornell University.

We have less control to move away our attention from half a conversation (or halfalogue) than when listening to a dialogue, said Lauren Eberson, a co-author of the study that will be published in the journal Psychological Science.

Since halfalogues really are more distracting and you can't tune them out, this could explain why people are irritated, she said in an interview.

Last year Americans spent 2.3 trillion minutes chatting on cellphones, according to the U.S. wireless trade association CTIA -- a ninefold increase since 2000.

Worldwide, there are about 4.6 billion cellphone subscribers, according to the International Telecommunications Union, a U.N. agency. The number is equal to about two-thirds of the world's population, leaving few corners of the globe where public spaces are free of mobile-tethered babblers.

China has the most cellphone users with 634 million, followed by India with 545 million and the United States with 270 million, figures from the U.S. Central Intelligence Agency (CIA) show.

Eberson said people try to make sense of snippets of conversation and predict what speakers will say next.

When you hear half of a conversation, you get less information and you can't predict as well, she said. It requires more attention.

The findings by Eberson and her co-author Michael Goldstein are based on research involving 41 college students who did concentration exercises, like tracking moving dots, while hearing one or both parties during a cellphone conversation.

The students made more errors when they heard one speaker's side of the conversation than when overheard the entire dialogue.

The study shows that overhearing a cellphone conversation affects the attention we use in our daily tasks, including driving, Eberson said.

These results suggest that a driver's attention can be impaired by a passenger's cell phone conversation, according to the study.

It recommends similar studies should be conducted with driving simulators.

(Editing by Patricia Reaney