Glenn Wilson, a former psychology lecturer at Gresham College in London, calls it infomania. His research found that being in a situation where you're trying to focus on a task, and you have an unread email in your inbox can reduce your effective IQ by 10 points. Russ Poldrack, a neuroscientist at Stanford, says that in the process of learning while multitasking new information is directed to the wrong part of the brain.

Then there are the metabolic costs. Asking the brain to shift attention from one activity to other forces the prefrontal cortex and striatum to burn oxygenated glucose, the same fuel they need to stay focused on tasks.

This leads to compromises in cognitive and physical performance. Among other things, repeated task switching leads to anxiety, which increases levels of the stress hormone cortisol in the brain, which in turn can lead to aggressive and impulsive behavior. Then there is the problem of the decisions to be taken which in multitasking is amplified dramatically causing a kind of short circuit.

One of the first things we lose is impulse control, a state of impoverishment quickly sets in where, after making a lot of meaningless decisions, we risk ending up making bad decisions about something important. So, to sum up, the use of multitasking causes an impoverishment of some of our cognitive abilities and more than decreasing intelligence (although Wilson's study has shown it) it leads to an increase in stress.