The researchers, Michael Platt and Allison McCoy, published their findings in the advanced online version of Nature Neuroscience, posted August 14, 2005. The research was sponsored by the National Institutes of Health, the EJLB Foundation, and the Klingenstein Foundation.
In their experiments, the researchers gave two male rhesus macaque monkeys chances to choose to look at either of two target lights on a screen. Looking at the "safe" target light yielded the same fruit juice reward each time. However, looking at the "risky" target light might yield a larger or smaller juice reward. The average juice reward delivered by looking at either target was the same.
To their surprise, the monkeys overwhelmingly preferred to gamble by looking at the risky target. This preference held, regardless of whether the scientists made the risky target reward more variable, or whether the monkeys had received more or less fruit juice during the course of the day.
"There was no rational reason why monkeys might prefer one of these options over the other because, according to the theory of expected value, they're identical," said Platt. The researchers also tested whether the monkeys were simply responding to the novelty of the risky target.
"We wondered whether the monkeys preferred the risky target because the experiment was dull and boring, and they wanted the variability," said Platt. "But when we made the task more interesting by changing the color of the lights on each trial, the monkeys didn't care anything about it."