What Gambling Monkeys Teach Us About Human Rationality

04 Feb

From the website Mind Hacks:

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.


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.

As the writer says, people being bad at probability might have some sort of primitive cause. A module or something that evolved in our brain before homo sapiens were sapient. If seeing consistency where there is none came about due to our evolutionary heritage, then things like believing in conspiracy theories or the supernatural were sort of bred into us by evolutionary processes. Combine this premise with our highly social brain and we might have another reason why belief in god is so prevalent, which is usually the go-to example of failing at using probability correctly.

Another commenter on the site provides some additional common irrationalities between us and other animals:

Humans also succumb to another fallacy that is strikingly irrational from an economic standpoint: They often give greater value to objects of good quality than to the same objects together with objects of lesser quality. This so-called “less is more effect” can be demonstrated when humans are asked to estimate the value of two alternatives, one of which is objectively of greater value than the other. For example, in one study subjects bid at an auction on 10 baseball cards in mint condition and at a different time on the same 10 cards with an additional 3 cards that were judged to be in poorer condition. Although the 3 cards in poorer condition were not worth as much as the cards in mint condition, they were each worth something. Nevertheless, the bid for the 10-card set was on average 59% higher than it was for the 13-card set.

Interestingly, animals, too, appear to experience this kind of sub-optimal judgment. For instance, monkeys willingly ate a piece of sliced vegetable or a grape but when offered a choice between them, showed a clear preference for the grape over the vegetable slice. However, surprisingly, when they were offered a choice between a single grape and a grape plus a slice of vegetable, they reliably preferred the single grape. This is hard to understand, as one would think that the struggle for existence teaches animals “every calorie counts”.

To test the generality of this effect, Zentall conducted a similar experiment with dogs. The dogs showed a preference for a small piece of cheese over a small piece of carrot but would willingly eat the piece of carrot when offered by itself . When, on critical test trials, the researcher offered the dogs a piece of cheese together with a piece of carrot versus a piece of cheese alone, all of the dogs except one preferred the piece of cheese alone.

We should keep stuff like this in mind when we get upset that people are behaving in irrational ways… like not vaccinating their children. Which is definitely another example of failing at applying probability correctly (probability is logic of science). We’re still animals. Social animals, but animals nonetheless.

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Posted by on February 4, 2015 in cognitive science


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