This article is reprinted with permission from The Ergonomics Report™ Archives, where it originally appeared on April 21, 2010.
Two  studies investigating multi-tasking made the headlines. In one, researchers found that people are generally limited to performing two mental tasks at any given time; the other found that most people exhibit significant performance decrements under dual task conditions, but that a small percentage of people, who the authors label "supertaskers," are able to perform two tasks at a time without any performance degradation.
French researchers Sylvain Charron and Etienne Koechlin, publishing in the journal Science (16 April 2010: Vol. 328. no. 5976, pp. 360 – 363), used FMRI (Functional Magnetic Resonance Imaging) to scan the brains of 32 test subjects while they performed letter matching tasks. They "observed that the left and right MFC [medial frontal cortex], which jointly drive single-task performance according to expected rewards, divide under dual-task conditions." That is, the two brain hemispheres cooperate when there is only one task, but divide and treat the tasks separately under dual task conditions. In general, subjects were accurate when performing two tasks, but performance suffered under three-task conditions.
The subjects in the experiment were motivated to perform through monetary rewards. According to Koechlin:
… the way we motivated subject is that for each task there was reward that they will earn only if they made no errors during a certain number of trials. So it was a monetary reward actually. So, whenever subjects made an error on one of the tasks, the reward was less.
Koechlin sums up the primary implication of the study, saying:
… the human prefrontal function seems to be unable to control the execution of more than two goals simultaneously. So it’s something which means that in everyday behavior, we can readily switch between two tasks, but not between three tasks.
Read the transcript or listen to a Podcast of this interview …
In the other study, University of Utah researchers Jason M. Watson and David L. Strayer tested 200 subjects in a driving simulator under both single- and dual-task conditions. The dual-task involved driving while concurrently performing "a demanding auditory version of the operation span (OSPAN) task," which was delivered through a cell phone. The driving task simulated traffic conditions that occasionaly required braking to avoid slowing cars ahead.
The majority of test subjects demonstrated poorer performance in both driving (braking reaction time and following distance) and OSPAN (memory and math) performance under dual task conditions. However, 2.5% of the subjects, labeled as "supertaskers' by the authors, "performed as well, if not better, in the dual-task condition than they did in the single-task conditions." They note that the supertaskers also performed in the upper quartile of the performance tests under single task conditions. As the authors put it:
… being good at the individual tasks was a necessary, but not sufficient condition for an individual to be classified as a supertasker. To be a supertasker, participants also had to show no dual-task costs.
Supertaskers have a strikingly remarkable ability to successfully perform two attention demanding tasks that over 97% of the population cannot perform without incurring substantial costs in performance. The behavioral characteristics of supertaskers are likely to be important in other activities that require coordinating a number of concurrent tasks (e.g., flying a high-performance aircraft) …
… our studies over the last decade have found that a great many people have the belief that the laws of attention do not apply to them (e.g., they have seen other drivers who are impaired while multi-tasking, but they are the exception to the rule). In fact, some readers may also be wondering if they too are supertaskers; however, we suggest that the odds of this are against them.
Read the full article …
These studies [came] on the heels of a 2009 multi-tasking study in which Stanford Universtiy researchers Ophir, Nass, and Wagner found that people who are regularly bombarded with several streams of electronic information do not pay attention, control their memory or switch from one job to another as well as those who prefer to complete one task at a time. In short, they found that people who perceived themselves as effective multi-taskers, and people who may also be perceived by others as "gifted" multi-taskers, actually perform worse than those who prefer to complete one task at a time.
As Ophir suggests:
They couldn't help thinking about the task they weren't doing. The high multitaskers are always drawing from all the information in front of them. They can't keep things separate in their minds. Read the full article …
The Bottom Line — How this Applies to Ergonomists
A primary goal in ergonomics is to understand human performance, and to apply that understanding to the design of equipment, tasks and systems.
Depending on the type of problem an ergonomist is solving, measures of human performance might include the time it takes people to complete a given task (productivity), their accuracy or reliability while performing that task (error making, quality), and their safety (injury avoidance and accommodation).
Some ergonomists focus primarily on the physical nature of a task, looking at physical loading risk factors like force, posture, repetition and duration, but tend to overlook mental loading, or cognitive ergonomics issues. There are many jobs that require multi-tasking, both physical and cognitive. Complex systems, such as the control rooms in an oil refinery or power plant, the cockpit of a jet aircraft, and even an automobile, all require performance during a complex mix of tasks, and the human loading is often more cognitive than it is physical in these environments.
The implications are clear for activities such as driving while texting or talking on a mobile phone. But what about the nurse or doctor working in a busy hospital environment, attempting to juggle multiple patients and multiple tasks? What about busy production staff overseeing multiple jobs, tasks and customers? What about typical office environments where computer users must manage multiple tasks, using multiple software programs, under demanding performance conditions? These and other typical work environments also demand attention to cognitive demands. In addition to evaluating physical factors in a system, ergonomists must also consider cognitive loading, including the potential effects of multi-tasking.
Don't design for the rare supertaskers; do design for the rest of us.