…in addition to slowing you [mentally] down, multitasking lowers your IQ. A study at the University of London found that participants who multitasked during cognitive tasks experienced IQ score declines that were similar to what they’d expect if they had smoked marijuana or stayed up all night. IQ drops of 15 points for multitasking men lowered their scores to the average range of an 8-year-old child.
That strikes me as correct. But I view this with a healthy dose of skepticism:
Researchers at the University of Sussex in the UK compared the amount of time people spend on multiple devices (such as texting while watching TV) to MRI scans of their brains. They found that high multitaskers had less brain density in the anterior cingulate cortex, a region responsible for empathy as well as cognitive and emotional control.