Scientists using sophisticated psychological tests and brain-imaging technologies have reached this conclusion: It is hard to rub your stomach and pat your head at the same time.
So is taking an order while you're deciding which restaurant table needs more coffee or talking on the phone with a client as you check your stocks online.
It turns out a chronic fact of 21st-century life — multitasking — actually, gasp, might be a waste of time.
"A lot of time people have illusions about what is possible or impossible," said David Meyer, a professor of mathematical psychology at the University of Michigan and co-author of a study on multitasking published yesterday in the Journal of Experimental Psychology: Human Perception and Performance. "That may lead you to believe you have abilities or limitations that don't really apply."
His conclusion is that rapidly switching between tasks wastes time, sometimes a lot of it.
Meyer and his colleagues had young adults perform tasks of varying degrees of familiarity and complexity, such as alternately doing math problems and identifying geometric shapes. The researchers measured the time it took to complete the tasks.
"One of the surprises was the magnitude of time costs involved in task-switching," Meyer said.
In some cases, multitasking added 50 percent to the time required to perform the chores.
The volunteers accomplished the tasks more slowly and did them more poorly, even when offered cash for jobs well done.
"Not only speed of performance, the accuracy of performance, but what I call the fluency of performance, the gracefulness of the performance, was negatively influenced," Meyer said.
Similarly, scientists at Carnegie Mellon University in Pittsburgh using brain-imaging technology found that the level of brain activity devoted to a task decreases when two tasks are performed at the same time.
They report in the journal NeuroImage that people doing two tasks at once do neither as well as when doing one at a time.
Meyer said some people can perform tasks simultaneously and save time, particularly if the tasks are familiar. But for most, time-saving is an illusion, he said.
The Michigan study, paid for by the Navy and conducted in part by the Federal Aviation Administration, could lead to better design of crucial jobs, such as air traffic control and vehicle operation, Meyer said.