One sunny July afternoon in a San Francisco park, tech recruiter Phil Tirapelle was tapping away on his cellphone while walking with his 18-month-old son. As he was texting his wife, his son wandered off in front of a policeman who was breaking up a domestic dispute.

"I was looking down at my mobile, and the police officer was looking forward," and his son "almost got trampled over," he says. "One thing I learned is that multitasking makes you dumber."

Yet...