When given a choice between steady rewards and the chance for more, monkeys will gamble, a new study found.
And they'll keep taking risks as the stakes rise and dry spells get longer.
The research, in which scientists also pinpointed brain activity during the gambling, could provide insight into the human penchant for risk.
The male rhesus macaque monkeys were shown either of two lights on a screen. Looking at a "safe" light yielded the same fruit juice reward each time. Looking at the "risky" light meant a larger or smaller juice reward. In the first test, the average reward was the same over time regardless of which light they chose.
The monkeys overwhelmingly preferred to gamble, even when the game was changed so that gambling yielded less juice over time.
"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 Duke University Medical Center neurobiologist Michael Platt.
Okay fine. So let's change the odds. In test two, the researchers made the average payoff for the risky target less than for the safe target.
"We found that they still preferred the risky target," Platt said. "Basically these monkeys really liked to gamble."
Platt and his colleague, Allison McCoy, fiddled with the odds even more, forcing a string of losses. But something kept the monkeys going, they reported in the Aug. 14 issue of the journal Nature Neuroscience.
"It seemed very, very similar to the experience of people who are compulsive gamblers," Platt points out. "While it's always dangerous to anthropomorphize, it seemed as if these monkeys got a high out of getting a big reward that obliterated any memory of all the losses that they would experience following that big reward."
The researchers then wired electrodes into a part of the monkeys' brains that, in humans and animals, is known to process information on rewards.
"As we increased the riskiness of a target, the neurons' activity would go up in the same way the monkey's frequency of choosing that target would go up," Platt said. "It was amazing the degree to which the activity of these neurons paralleled the behavior of the monkeys. They looked like they were signaling, in fact, the monkeys' subjective valuation of that target."
More work is needed to map the entire circuitry involved in the process, the scientists say. Then, studies could possibly be done to how brains process risk and reward.
In humans, it's thought that low levels of the neurotransmitter serotonin might make one more risk-prone and impulsive. Perhaps, the scientists say, future work will shed light on the source of pathological gambling, obsessive-compulsive disorder and even depression.