Many executives see themselves as prodigious multitaskers -- checking e-mail while simultaneously talking on the phone, eyeing stock quotes, and performing all sorts of other busywork. But they're probably doing a lousy job if they're trying to focus on more than four things at once, a recent study maintains.

Researchers at the University of Oregon have concluded that the human brain has a built-in limit on the number of discrete thoughts it can entertain at one time. The limit for most individuals is four, according to the research team led by University of Oregon psychology professors Edward Awh and Edward Vogel.

The multitasking cap is not affected by the complexity of one's thoughts. Surprisingly, as many complex concepts can be retained in short-term memory as simple thoughts, the researchers found.

That makes the human brain dissimilar to an electronic gadget like an iPhone, they said. A 4 GB iPhone can hold about 1,000 four-minute songs. But fewer tunes can be stored if they're longer and consume more memory space.

Awh and Vogel's team also found a strong link between short-term memory capacity and intelligence. "People with high IQs can think about more things at once," according to the study.

But it's no sure bet that such individuals will have an easier time passing tests or succeeding on the job. Their memories may be capacious -- but not necessarily clear. "Even though people with high IQs can think about more things at once, there are no guarantees about how good those memories might be," said Awh, in a statement.

The study was funded by the National Institutes of Health and the National Science Foundation. The results were recently published in the journal Psychological Science.