So hypothetically, we have a dilemma.

You’ve been arrested and imprisoned with your partner in crime... me.

But it turns out the police don’t have a lot of evidence, so they separate us and offer you a bargain deal: You deny the crime and pin it on me, and you walk free.

The police say I’ve been offered the same bargain, but you have no way of knowing my response.

And… there’s a catch.

If you deny the crime and say I’m guilty, you walk free but I’m in prison for three years.

And I could deny the crime and pin it on you – but then you’re in prison for three years.

If we both blame each other, we each serve two years in prison.

And if we both deny and stay quiet, we each serve one year in prison.

So, what do you do?

This is the Prisoner's Dilemma, it’s a paradox in Game Theory, the study of strategic decision making.

The dilemma is designed to see whether you cooperate with the other person or not.

In economics it’s called a simultaneous game – your decision is based on what you expect my decision to be.

Just like rock, paper, scissors.

Hypothetically, if I wanted the best outcome for myself, I’d rat you out.

Sorry!

I’d get to walk free!

There is an advantage to cooperating, but there’s a bigger personal advantage to being selfish.

But in lots of studies using the prisoner’s dilemma, the most common response is to cooperate with the other person, so both stay quiet and serve a year in prison.

And in the real world, cooperation exists between people, even when it doesn’t lead to an immediate or direct reward.

To economists, the prisoner's dilemma is a paradox because individually rational behaviour–you walking free–leads to collectively irrational results–your partner getting more jail time.

Imagine it this way: One day you catch most of the trout from a lake and sell it.

This has a great short term benefit for you, but the strain of you overfishing the lake affects all of its users – it’s common property.

This is known as the tragedy of the commons.

It’s better if you cooperate with all the users, and personally, you’ll do better over time.

The thing is, we don’t think about the consequences over time in a one-off game like the prisoner’s dilemma.

In another version of the dilemma, users face the problem time and time again, and with the same person.

And come to think of it, I don’t want to be back in the same situation after ratting you out.

In this case, economists say the best individual strategy is tit-for-tat: you cooperate the first time and then do whatever the other person did the last time.

The threat of retaliation makes us much less likely to be a jerk.

We even see tit-for-tat strategies in animals – female vampire bats will regurgitate their bloody food for other female bats that couldn’t find food, but only if those bats have helped them in the past.

Over time, everyone does better if they cooperate.

But why is that?

Why do we cooperate if the principle is sometimes counterintuitive to our survival?

Well, social cooperation is intrinsically rewarding to our brains.

In one study, participants did the prisoner’s dilemma with an assumed human participant in an fMRI machine.

When people cooperated in the dilemma, activation was seen in brain areas linked to reward processing.

The researchers suggest this activation of the reward circuit may help to override the temptation people get to not cooperate.

The researchers suggest this activation of the reward circuit may help to override the temptation people get to not cooperate.

More brain imaging research found that when people cooperate in the prisoner’s dilemma, it increased the firing frequency of dopamine neurons in the midbrain.

And they decreased in firing frequency when people didn’t cooperate and return the favour.

These neural mechanisms help us learn who is trustworthy in reciprocating favours, and who is not.

These studies have been done with university students, and that one with the vomiting female vampire bats.

The prisoner’s dilemma was originally conceived in 1950, but no one thought to do the test on actual prisoners for over 60 years.

So in 2013, German researchers found that inmates were actually more likely to cooperate with other inmates than university students were with other students.

Think back to your answer at the beginning of this video – if you were in a real life situation, do you think your answer would change?

Social decision making is one of the most complex human behaviours.

When you think about it, you have to consider what other people are deciding and adjust your behaviour; and so many other factors, like context, influence people’s response.

More than anything, it’s hard to predict how much people are willing to sacrifice their personal gains to increase or decrease the well-being of others.

Did you?