A study indicates that multitasking may actu-ally 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.

Researchers 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 clas-sifying 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 loss increased with the complexity and the unfamiliarity of the tasks.

Drawing from their research, researchers propose two stages 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.

Accessing the rules for the new task and activating them 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. Those lost seconds can also build up for air traffic controllers, pilots and office workers surfing the Web while writing a report.