False. 
There has been a fair amount of research performed on multitasking. However, before I reference a scientific study I’d like to discuss an “informal experiment” I conducted with one of my best friends in a canoe on a sunny summer afternoon. My friend and I were fishing, and the fish were biting! My friend has the habit of using 2 or 3 rods at the same time, and this particular day he was using 3. He held one in his hand, had one balanced in his lap, and had the other one propped up diagonally in the canoe. Not paying close attention to any of his rods, he missed many more fish than he caught. I’d estimate I out-fished him 5 to 1 that afternoon. That was all the “evidence” I needed to confirm that it is better to focus on one task than to do multiple things at once. Our brains are wired in such a way as it is difficult for us to take in multiple streams of information at one time. Likewise, we are not wired to be able to perform more than one task at a time very well (try reading and saying the alphabet at the same time). Now you might think that, considering the lives many of us lead, multitasking seems to be a necessity. A vision of a mother paying the family bills, checking her son’s homework, cooking dinner, answering e-mails, talking on the phone, all while listening to her i-pod, comes to mind. The truth is that when we try and do two things at once, our productivity actually decreases. The authors of one study (Ophir et. al., 2009) which examined cognitive control in media multitakers concluded “This led to the surprising result that heavy media multitaskers performed worse on a test of task-switching ability”. Not only can multitasking decrease productivity, it can also be dangerous – think about the dangers of texting while driving!
Reference:
Ophir E, Nass C. Wagner A: Cognitive control in media multitaskers. Psychological and Cognitive Sciences (2009), Vol 106, pps. 15583-15587.