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Doing Two Things at the Same Time

How many things can we do at once? Fewer than we think, say psychologists, who are identifying “bottlenecks” in the process.

People routinely do two or more things at the same time. They eat breakfast while reading the newspaper; they make calls on a cellulartelephone while driving a car; and, of course, they chew gum while walking. If you ask people to assess their own ability to keep two activities going at once, they generally report difficulty only if one of the tasks is intellectually demanding; for example, they may have a hard time carrying on a serious discussion while adding up a restaurant check. If the tasks are routine, people are quite sure they can handle at least two simultaneously. Recent research suggests that these assessments of the human capacity for parallel processing may be over-optimistic.

It appears that certain mental operations are “bottlenecks” that require the exclusive use of some cognitive resource and therefore cannot be done concurrently. In particular, even the most trivial forms of decision-making and memory retrieval seem to be activities that cannot be over- lapped with other operations. Much of what people perceive as parallel processing in mental life may really be more like computer time sharing, in which some mental operations are actually carried out one at a time.