http://discovermagazine.com/2005/aug/email-make-you-dumber

A recent study for the company found that British workers’ IQ test scores drop temporarily by an average of 10 points when juggling phones, e-mails, and other electronic messages–more of an IQ drop than occurs after smoking marijuana or losing a night’s sleep. “This is a very real and widespread phenomenon,” said Glenn Wilson of the Institute of Psychiatry at the University of London, who conducted the tests on some 1,100 volunteers. Just how long it takes to recover is unclear.