How Ambiguity Messes with Our Brains
Red and black ace cards.

The brain responds emotionally and often illogically when forced to make decisions based on little or conflicting evidence, a new study suggests.

These types of choices are known as ambiguous decisions and are different from risky decisions.

In a risky decision, a person is uncertain about the outcome of their choice but has an idea of the probability of success. In an ambiguous decision, a person is ignorant of both factors.

"Psychologists would say ambiguity is the discomfort from knowing there is something you don't know that you wish you did," said Colin Camerer, an economist at the California Institute of Technology and the primary researcher in the study.

In the experiment, test subjects made ambiguous bets while their brains were scanned using a functional magnetic resonance imager (fMRI).

In one example, the subjects were given the choice between betting money on the chances of drawing a red card from a "risky" deck that had 20 red cards and 20 black cards??"that is, where the probability of choosing either color was 50:50??"and making the same bet with an "ambiguous" deck where the color composition of the cards was unknown.

In most cases, the subjects chose to make the risky bet. Logically, however, both bets would have been equally good because in both cases, the chance of pulling a red card on the first draw was 50:50.

The brain scans revealed that ambiguous wagers were often accompanied by activation of the amygdala and orbitofrontal cortex (OFC), two areas of the brain that are involved in the processing of emotions. In particular, the amygdala has been found to be closely associated with fear.

A correlation between aversion to ambiguous decisions and activation of emotional parts of the brain makes sense from an evolutionary point of view, Camerer said. "Freezing in the face of danger is an old, emotional response which probably was evolutionarily adaptive in our ancestral past."

In the modern human brain, this translates into a reluctance to bet on or against an event if it seems at all ambiguous.

The finding could help scientists understand how humans make decisions in the real world, because the choices people make are often based on very limited information, Camerer told LiveScience.

"If you think about it, how often do you know the probability of success?" he said. "Probably, the situation we modeled with the risk game is more the exception than the rule."

The study was detailed in the Dec. 9 issue of the journal Science.