Two At a Time for the Brain

Many people believe they can ‘multitask’ very efficiently. Those who do, may need to think again. Studies by researcher researcher Etienne Koechlin of the Université Pierre et Marie Curie in Paris, France show that “… we can readily divide tasking. We can cook, and at the same time talk on the phone, and switch back and forth between these two activities. However, we cannot multitask with more than two tasks.” According to Koechlin, the study results might also explain why humans seem to have difficulty when decisions involve more than two choices. When faced with three or more choices, subjects don't appear to evaluate them rationally; they simply start discarding choices until they get back to a binary choice. This is perhaps because your brain can't keep track of the rewards involved with more than two choices.

http://www.scientificamerican.com/article/multitasking-two-tasks/