When we try to do two things at once, each half of the brain focuses on a separate task, French scientists say.
This division of labour could explain why we find it so difficult to multi-task, they report in the journal Science.
It might also explain why people are prone to make irrational decisions when choosing from a long list of items.
Lead author Dr Etienne Koechlin told the BBC: “You can cook and at the same time talk on the phone but you cannot really do a third task such as trying to read a newspaper.
“If you have three or more tasks you lose track of one task.”
The French team used an imaging technique to monitor brain activity in 32 volunteers asked to perform a letter-matching test.
The scans looked at the frontal cortex, the part of the human brain associated with impulse control.
When the volunteers completed one task at a time, one side of a certain area of the frontal lobes lit up.
But, when they completed two tasks at the same time, the lobes divided the tasks between them.
Activity on the left frontal lobe matched the primary task (action A) and activity on the right corresponded to the secondary task (action B).
The brain was able to control switching between the two hemispheres when carrying out dual functions but accuracy suffered when a third was added.
Dr Koechlin, of Ecole Normale Superieure in Paris, France, said this behaviour could explain why people make “some irrational decisions” when faced with more than two choices.
“My view is that it’s critically related to this division of labour between the two hemispheres to keep track of two tasks or two options but not more,” he explained.
“Our result is likely to provide an explanation for why people are good in binary choice but not multiple choice.”