What gambling monkeys teach us about human rationality

We often make stupid choices when gambling, says Tom Stafford, but if you look at how monkeys act in the same situation, maybe there’s good reason.

When we gamble, something odd and seemingly irrational happens.

It’s called the ‘hot hand’ fallacy – a belief that your luck comes in streaks – and it can lose you a lot of money. Win on roulette and your chances of winning again aren’t more or less – they stay exactly the same. But something in human psychology resists this fact, and people often place money on the premise that streaks of luck will continue – the so called ‘hot hand’.

The opposite superstition is to bet that a streak has to end, in the false belief that independent events of chance must somehow even out. This is known as the gambler’s fallacy, and achieved notoriety at the Casino de Monte-Carlo on 18 August 1913. The ball fell on black 26 times in a row, and as the streak lengthened gamblers lost millions betting on red, believing that the chances changed with the length of the run of blacks.

Why do people act this way time and time again? We can discover intriguing insights, it seems, by recruiting monkeys and getting them to gamble too. If these animals make dumb choices like us, perhaps it could tell us more about ourselves.

First though, let’s look at what makes some games particularly likely to trigger these effects. Many results in games are based on a skill element, so it makes reasonable sense to bet, for instance, that a top striker like Lionel Messi is more likely to score a goal than a low-scoring defender.

Yet plenty of games contain randomness. For truly random events like roulette or the lottery, there is no force which makes clumps more or less likely to continue. Consider coin tosses: if you have tossed 10 heads in a row your chance of throwing another heads is still 50:50 (although, of course, at the point before you’ve thrown any, the overall odds of throwing 10 in a row is still minuscule).

The hot hand and gambler’s fallacies both show that we tend to have an unreasonable faith in the non-randomness of the universe, as if we can’t quite believe that those coins (or roulette wheels, or playing cards) really are due to the same chances on each flip, spin or deal.

It’s a result that sometimes makes us sneer at the irrationality of human psychology. But that conclusion may need revising.

Cross-species gambling

An experiment reported by Tommy Blanchard of the University of Rochester in New York State, and colleagues, shows that monkeys playing a gambling game are swayed by the same hot hand bias as humans. Their experiments involved three monkeys controlling a computer display with their eye-movements – indicating their choices by shifting their gaze left or right. In the experiment they were given two options, only one of which delivered a reward. When the correct option was random – the same 50:50 chance as a coin flip – the monkeys still had a tendency to select the previously winning option, as if luck should continue, clumping together in streaks.

The reason the result is so interesting is that monkeys aren’t taught probability theory as school. They never learn theories of randomness, or pick up complex ideas about chance events. The monkey’s choices must be based on some more primitive instincts about how the world works – they can’t be displaying irrational beliefs about probability, because they cannot have false beliefs, in the way humans can, about how luck works. Yet they show the same bias.

What’s going on, the researchers argue, is that it’s usually beneficial to behave in this manner. In most of life, chains of success or failure are linked for good reason – some days you really do have your eye on your tennis serve, or everything goes wrong with your car on the same day because the mechanics of the parts are connected. In these cases, the events reflect an underlying reality, and one you can take advantage of to predict what happens next. An example that works well for the monkeys is food. Finding high-value morsels like ripe food is a chance event, but also one where each instance isn’t independent. If you find one fruit on a tree the chances are that you’ll find more.

The wider lesson for students of human nature is that we shouldn’t be quick to call behaviours irrational. Sure, belief in the hot hand might make you bet wrong on a series of coin flips, or worse, lose a pot of money. But it may be that across the timespan in evolution, thinking that luck comes in clumps turned out to be useful more often than it was harmful.

This is my BBC Future article from last week. The original is here