New scientific studies reveal the hidden costs of multitasking as technology increasingly tempts people to do more than one thing at a time. Whether people toggle between browsing the Web and using other computer programs, talk on cell phones while driving, pilot jumbo jets or monitor air traffic, they’re using their“executive control” processes—the mental CEO—found to be associated with the brain’s key neural regions. These interrelated cognitive processes establish priorities among tasks and allocate the mind’s resources to them. “People have specific mental resources whose effective use requires supervision through executive mental control,” says David Meyer, Ph. D. at the University of Michigan.

To better understand executive control, as well as the human capacity for multitasking and its limitations, Joshua Rubinstein, Ph. D. of U. S. Federal Aviation Administration, and David Meyer, studied patterns in the amounts of time lost when people switched repeatedly between two tasks of varying complexity and familiarity. The findings revealed that for all types of tasks, subjects lost time when they had to switch from one task to another, and time costs increased with the complexity of the tasks, so it took a significantly long time to switch between more complex tasks. Time costs were a lot greater when subjects switched to tasks that were relatively unfamiliar. They got “up to speed” faster when they switched to tasks they knew better, an observation that may lead to interfaces designed to help overcome people’s innate cognitive limitations.

The researchers say their results suggest that executive control involves two distinct, complementary stages: goal shifting (“I want to do this now instead of that”) and rule activation (I’m turning off the rules for that and turning on the rules for this”). Both stages help people unconsciously switch between tasks. Rule activation itself takes significant amounts of time, several tenths of a second—which can add up when people switch back and forth repeatedly between tasks. Thus, multitasking may seem more efficient on the surface, but may actually take more time in the end. According to the researchers, this insight into executive control may help people choose strategies that maximize their efficiency when multitasking. The insight may also weigh against multitasking. For example, Meyer points out, a mere half second of time lost to task switching can mean the difference between life and death for a driver using a cell phone, because during the time that the car is not totally under control, it can travel far enough to crash into obstacles that the driver might have otherwise avoided.