Have you ever been talking to someone on the telephone and gotten the distinct impression that the person on the other end of the line was reading e-mail or surfing the Web during the conversation?

Vague, somewhat random responses like “ummm-humm” and “I see,” coupled with occasional mouse clicks, are telltale signs you may not have the listener’s undivided attention.

It turns out that such behavior, which the practitioners refer to as multitasking, isn’t only annoying and rude, it’s inefficient. That’s because the person trying to do two things at once probably isn’t doing either of them very well.

That goes against the grain of a society that seems increasingly enamored of multitasking, which has been made more possible, even unavoidable, by computers, cell phones, IPODs and television.

The problem with multitasking is that the human brain, despite its almost unimaginable cognitive power, has a basic limitation. It is unable to completely concentrate on more than one thing at a time.

Researchers at Vanderbilt University asked study participants to do a simple cognitive task, which they performed easily. Then a second simple task was added to the first, to be completed almost simultaneously. The participants’ response to the additional task was delayed by about a second, compared to the speed with which they completed each task alone.

One second may not seem like much, but it’s magnified when the tasks become more complex. On a highway, it can be the difference between a close call and a tragedy.