To take an example, I made our Friday night pizzas tonight while having a lovely conversation with our twelve-year-old son. Since my hands were doing one thing while my mouth was doing another, you might say I was simultasking. But I’ve made pizzas so often (we have homemade pizza every other Friday) that it doesn’t take a whole lot of brain power to roll out the dough or sprinkle shredded cheese on top of the tomato sauce. That left a whole lot of brain cells free to do the listening and speaking bits. It wouldn’t have been nearly as interesting a conversation if I had been checking the bank statement, which would have required a lot more attention.
I’ve seen a theory that traditional “women’s work”, such as gathering food and spinning thread, became women’s work specifically because it was so easy to combine with child minding. (Child minding was obviously women’s work because the children small enough to be nursing had to stay with Mom.) You can’t take a three-year-old along on a mammoth hunt or to a smithy, but you can keep him in sight while you weed the garden or weave a basket - and if necessary put your work down to keep him from wandering off.
If you think of brain power the same way we think about computing power, this argument becomes much clearer.
Time division multi-tasking (switching back and forth between tasks) is inherently less efficient than doing each task one-at-a-time, due to the context switching overhead (i.e. switching from amount of sauce on pizza and cheese coverage to Timmy’s tone of voice, how his vocabulary translates into concepts I understand, and whether or not he’s trying to sneak something past me). However, if one spends too long on one task we can lose interest and become less efficient as a result. Thankfully, most tasks we’re faced with get resolved before we hit that drop-off point.
One aspect to keep in mind is that certain repetitive tasks form part of procedural memory and thus (as far as I know) leave more working memory free for other tasks. Thus I can listen to music, play a game and browse the Straight Dope or an article (if it’s not too taxing). I have to quit playing if I want to focus on typing though.
There’s some evidence that we can multitask as long as the same brain centers are not used. Richard Feynman talked about his experiments with counting with a consistent timing and doing other tasks, such as folding laundry, or talking, or reading, etc.
What he found was that there are some things he could do and some not, and it seemed to be associated with the parts of the brain in use. He couldn’t speak out loud while counting, for instance, because when he “spoke” the numbers inside his head the two things interfered.
However, he also found that other people had different limitations; a mathematician friend had a different method of counting that was more “visual”, and that he could speak while counting just fine but he couldn’t read.
I’d guess that any complicated task uses enough of the different brain centers that they can’t be multitasked with anything beyond the most trivial things. But it may be that two non-trivial tasks can be done simultaneously as long as they don’t overlap.
When I was a secretary, I used to take shorthand and type up letters from my notes. My boss would come in to discuss something, and I would be typing away AND talking to him at the same time. Not only talking, but looking right at him. It would drive him nuts. “How do you DO that??”
I dunno, boss, (type type type), you were saying about the meeting? (type type type).
Some bozo raised the issue publicly, and Cecil clued him in to leave it alone:
"…the myth of an innate female gift for multitasking serves two socially useful purposes: it enables women to rationalize having gotten stuck with the scutwork, while for you it’s an excuse to avoid helping out.
— Cecil Adams"
Set the issue down on the ground men, gently, and slowly back away…