THAT'S THE WHY:"I'm on the bus."

It’s often the prelude to a loud, mundane and frankly irritating half-conversation that commuters within earshot have to witness.

Whether it’s the minutae of the person’s oh-so-interesting day, blow-by-blow details of some argument being rehashed with dramatic effect or just other inane waffle, it’s the kind of anti-social annoyance that can turn a pleasant journey into an endurance test.

Why is it that overhearing someone talking on a mobile phone is so annoying and difficult to tune out?

It’s partly down to the unpredictability of a “half-heard” conversation, according to a study published last year in the journal Psychological Science.

The research played recorded “halfalogues”, or conversations where only one person could be heard, and tested the cognitive abilities of volunteers as they worked on other tasks that needed attention, like tracking a moving dot with a computer mouse.

The unpredictability of the one-sided conversations had a negative impact on the study volunteers’ ability to focus. But if both sides of the conversation were audible, volunteers were better able to concentrate on the tasks in hand.

“It’s definitely changed my own etiquette,” says Lauren Emberson, who carried out the research at Cornell University.

“I’m a lot more sensitive about talking on the phone in public. It has a really profound effect on the cognition of the people around you, and it’s not because they’re eavesdropping or they’re bad people.

“Their cognitive mechanism basically means that they’re forced to listen.”