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It reduces mental ability by an average of about 10 IQ points.
For men, it’s about three times the effect of smoking marijuana.
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.
And:
This 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 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.
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