“Almost everyone will nod when you say multitasking is inefficient, but no one will realize that serial tasking is just as bad. If you do Task A and switch to Task B, there’s a latency period where you find it very hard to switch,” said Sendhil Mullainathan, a professor of economics at Harvard. He is co-author, with Eldar Shafir, of “Scarcity: The New Science of Having Less and How It Defines Our Lives."