The key to intelligence may be the ability to juggle multiple thoughts or memories at one time.

Researchers have found that a simple test of working memory capacity strongly predicts a person's performance on a battery of intelligence tests that measure everything from abstract problem-solving to social intelligence.

Working memory is a way of temporarily storing information used for some mental task.

If the results of the study hold for the population at large, "I could predict an individual's overall intellectual ability essentially with 79-percent accuracy if you tell me what their working memory capacity is," said study researcher Steven Luck of the University of California, Davis.

Prior research suggests that since working memory can be improved, so can a person's intelligence.

Flashing colored squares

Luck and his colleagues used a working memory test they developed that asks subjects to recall the color of one of several colored squares flashed on a computer screen a few seconds before. By increasing the number of squares flashed onscreen, researchers can assess a person's ability to mentally store multiple visual objects – in this case, colors.

The purpose of the study, to be published in the June issue of the journal Archives of General Psychiatry, was to examine working memory deficits in people suffering from schizophrenia. Although the mental disorder is most well-known for its delusions and hallucinations, problems with thinking might ultimately be more important to understanding and treating the condition.

The researchers gave the working memory test to 31 schizophrenia sufferers and 26 control subjects of similar socioeconomic status, age and race. They also had subjects complete a series of intelligence tests known as the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) battery.

"[MATRICS] was designed to be used in testing the effects of new pharmacological treatments on cognition in schizophrenia, but it provides a broad measure of cognitive functioning in healthy individuals," Luck said.

Link between memory and IQ

The match between working memory capacity and MATRICS score was surprisingly strong in the control subjects, Luck said. "It is very rare to find a correlation that strong," he said.

"That's very unusual," agreed Nelson Cowan, a cognitive scientist at the University of Missouri, who was not involved in the study. "Almost nothing gives that high a correlation."

Cowan said the results indicate a connection between working memory and attention, because many of the tests in the MATRICS battery are related to a person's ability to keep track of multiple instructions at the same time.

"If you can't hold as many items in mind," Cowan said, "it may affect your ability to carry out complex procedures because the goals and the procedures themselves compete with items you are trying to remember."

The correlation between working memory and MATRICS score was much lower in people with schizophrenia. Luck said his next goal is to figure out why that is.

More memory slots

In a second new study, to be detailed in the journal Psychonomic Bulletin and Review, researchers from the University of Oregon also found a correlation between working memory capacity and intelligence scores in a group of healthy college students.

Working memory capacity isn't necessarily set in stone. There is evidence that people can improve their working memory – and possibly their intelligence – by practicing. In a 2008 study, people who trained on a demanding working memory task improved their scores on a simplified intelligence test by 20 percent, whereas people who didn't train improved by less than 10 percent.

People who have a high working memory capacity may simply be better at ignoring distractions.

"[They] may not have more memory slots than other people," Luck said. They may just be better at keeping relevant information in memory and irrelevant information out."