In Brief

When people do two things at once, are they being more efficient or wasting time? A study in the Journal of Experimental Psychology: Human Perception and Performance (Vol. 27, No. 4) indicates that multitasking may actually be less efficient--especially for complicated or unfamiliar tasks--because it takes extra time to shift mental gears every time a person switches between the two tasks.

To better understand the brain's "executive mental control"--or how it establishes priorities between tasks and allocates resources to them--researchers Joshua Rubinstein, PhD, of the Federal Aviation Administration, and David Meyer, PhD, and Jeffrey Evans, PhD, of the University of Michigan, conducted four experiments that measured the amount of time lost when young adults repeatedly switched between two tasks.

The tasks, such as solving math problems or classifying geometric objects, were either familiar or unfamiliar and simple or complex. For all types of tasks, participants lost time when they switched back and forth. Moreover, the time lost increased with the complexity and the unfamiliarity of the tasks.

Drawing from their research and previous studies, Rubinstein, Meyer and Evans propose a new model for executive mental control in which the brain must make two separate preparatory decisions to switch tasks. The first, called goal shifting, involves choosing to switch to a new task. The second, rule activation, requires the brain to turn off the cognitive rules of the old task and turn on the cognitive rules of the new task.

For example, a student who has completed her math homework and is ready to begin her English homework must first decide that she is done with math and ready to begin English (goal shifting) and then turn off the rules of addition and multiplication and activate the rules for reading a story (rule activation).

Accessing the rules for the new task and activating them, the researchers say, can take several tenths of a second--a significant amount of time for some tasks. For example, a mere half-second lost to task switching can be disastrous for a driver using a cell phone while maneuvering on a busy freeway, says Meyer. Those lost seconds can also build up for air traffic controllers, pilots and office workers surfing the Web while writing a report.

While the JEP study seems to indicate that multitasking isn't very efficient, its findings aren't definitive, according to another study in Psychological Science (Vol. 12, No. 2) by Eric H. Shumacher, PhD, of the University of California, Berkeley, and colleagues. That study found that, under special circumstances, people can do two different tasks at once without much interference--particularly if the tasks are well-practiced and do not physically conflict with each other.

"These latter results raise a further question--what causes you to get interference between tasks in some cases but not others?" asks Meyer, a co-author on both studies. "This is a challenging question and one that needs to be answered carefully in order to tell the full story about multitasking and executive control."