Can you really do two things at once?

Mostly, no. What feels like doing two things at once is usually your brain switching rapidly between them, and every switch has a price. Psychologists have measured it for nearly a century: you get slower and make more mistakes on a task you have just switched to than on one you were already doing. That penalty is called the switch cost, and it doesn't go away even when you know the switch is coming.

The psychologist David Meyer put it bluntly: when people alternate rapidly between tasks, errors go way up and the work often takes double the time or more than doing the jobs one after the other. With David Kieras he found that during the gap between each switch, the brain makes no progress at all, because it has to restart and refocus. Researchers think there is a bottleneck, especially for planning actions and pulling information from memory, so two demanding tasks end up queuing for the same narrow door.

The costs are clearest on the road. A 2006 study found that drivers talking on cell phones were more involved in rear-end collisions, and accelerated more sluggishly, than drunk drivers. In classrooms, students who multitask learn new information less well.

The twist is that most of us think we're the exception. People's sense of being good at multitasking, or of getting more done that way, is frequently wrong. A small group of 'supertaskers' really does seem to cope, but they are rare. And the popular idea that women multitask better than men has little data behind it.