Doing two things at once can make you "as dumb as a stoner"

from delancyplace.com:

In today's selection -- you can't do two things that require concentration at once
-- or at least you can't do them very well. And doing too much, even if not all
at once, has a debilitating effect:
"The idea that conscious processes need to be done one at a time has been studied
in hundreds of experiments since the 1980s. For example, the scientist Harold Pashler
showed that when people do two cognitive tasks at once, their cognitive capacity
can drop from that of a Harvard MBA to that of an eight-year-old. It's a phenomenon
called dual-task interference. In one experiment, Pashler had volunteers press one
of two keys on a pad in response to whether a light flashed on the left or right
side of a window. One group only did this task over and over. Another group had
to define the color of an object at the same time, choosing from among three colors.
These are simple variables: left or right, and only three colors. Yet doing two
tasks took twice as long, leading to no time saving. This finding held up whether
the experiment involved sight or sound, and no matter how much participants practiced.
If it didn't matter whether they got the answers right, they could go faster. The
lesson is clear: if accuracy is important, don't divide your attention.
"Another experiment had volunteers rapidly pressing one of two foot pedals to represent
when a high or low tone sounded. This exercise took a lot of attention. When researchers
added one more physical task, such as putting a washer on a screw, people could
still do it, sort of, with around a 20 percent decrease in performance. Yet when
they added a simple mental task to the foot-pedal exercise, such as adding up just
two single-digit numbers, (a simple 5 + 3 = ), performance fell 50 percent. This
experiment revealed that the problem isn't doing two things at once so much as
doing two conscious mental tasks at once, unless you are okay with a significant
drop in performance. ...
"Despite thirty years of consistent findings about dual-task interference, many
people still try to do several things at once. Workers of the world have been told
to multitask for years. Linda Stone, a former VP at Microsoft, coined the term continuous
partial attention in 1998. It's what happens when people's focus is split, continuously.
The effect is constant and intense mental exhaustion. As Stone explains it, 'To
pay continuous partial attention is to keep a top-level item in focus, and constantly
scan the periphery in case something more important emerges.'
"A study done at the University of London found that constant emailing and text-messaging
reduces mental capability by an average of ten points on an IQ test. It was five
points for women, and fifteen points for men. This effect is similar to missing
a night's sleep. For men, it's around three times more than the effect of smoking
cannabis. While this fact might make an interesting dinner party topic, it's really
not that amusing that one of the most common 'productivity tools' can make one as
dumb as a stoner. (Apologies to technology manufacturers: there are good ways to
use this technology, specifically being able to 'switch off' for hours at a time.)
'Always on' may not be the most productive way to work. One of the reasons for this
will become clearer in the chapter on staying cool under pressure; however, in summary,
the brain is being forced to be on 'alert' far too much. This increases what is
known as your allostatic load, which is a reading of stress hormones and other factors
relating to a sense of threat. The wear and tear from this has an impact. As Stone
says, 'This always on, anywhere, anytime, anyplace era has created an artificial
sense of constant crisis. What happens to mammals in a state of constant crisis
is the adrenalized fight-or-flight mechanism kicks in. It's great when tigers are
chasing us. How many of those five hundred emails a day is a tiger?' "
Author: David Rock
Title: Your Brain at Work
Publisher: Harper Business
Date: Copyright 2009 by David Rock
Pages: 35-36

Your Brain at Work: Strategies for Overcoming Distraction, Regaining Focus, and
Working Smarter All Day Long
by David Rock by HarperBusiness