Your Brain on Multitasking
We don't know enough about how the brain works to understand what happens when we do more than one thing at a time, but knowing how computers operate might provide the basis for an educated guess. A CPU executes one instruction at a time, but it swaps programs in and out several times a second, paying attention to each one for only a few milliseconds. This process, called context switching, always costs time, but data is never lost because the computer saves all the information it needs to get right back to what it was doing.
Our brains probably aren't set up that way. When we get distracted, we frequently have trouble remembering what we were doing. When we switch context, we lose both time and information.
Sometimes that loss doesn't matter. For example, when I was working the cash register at the Orange Julius stand, my brain was never a bottleneck in the process of giving people what they wanted to eat in exchange for their money. Losing a little time and concentration in the switch didn't matter; more important was maximizing the use of our most limited resources: the oven and the drink machine.
Other, more difficult activities can be done simultaneously. For instance, professional interpreters know that it's possible to listen in one language while talking in a different language. This technique probably works because the meaning of the words they hear is the same as what they're saying. It's doubtful that you can take in one idea while verbalizing a different one.
Some of the latest research on how the brain works indicates that we run into trouble when we try to use the same part of the brain for more than one thing at a time. If you've ever tried writing email on one subject and talking on the phone about something entirely different, you'll find that you simply can't do it. That's because the two activities require the same mental resources.
Another situation where it's hard to concentrate is when you're attempting to hold a conversation with somebody who isn't in the same physical context as you. This is why in many countries it's illegal to talk on your cell phone while driving. Even if you use a hands-free kit and aren't physically bothered, you are mentally distracted by the conversation. Talking to a passenger isn't the same as talking with somebody who isn't in the vehicle with you; because the faraway person isn't experiencing the same things as you are, you can't pace your conversation in the same way. Studies show that if you're talking on the phone while driving, your reaction time is diminished as much as if you were legally drunk.
This brings me to my favorite anecdote about the effects of doing several things at once. A study in the UK is said to show that multitasking lowers your IQ more than smoking marijuana would do. While this particular experiment wasn't scientifically rigorous, it made good press, and the headlines brought home an important point: If any task requires your concentration, isolate that task—don't multitask when you're doing it.