Studying human behaviour can be like a dog trying to catch its own tail. As we learn more about ourselves, our new beliefs change how we behave. Research on economics students showed this in action: textbooks describing facts and theories about human behaviour can affect the people studying them.
Economic models are often based on an imaginary character called the rational actor, who, with no messy and complex inner world, relentlessly pursues a set of desires ranked according to the costs and benefits. Rational actors help create simple models of economies and societies. According to rational choice theory, some of the predictions governing these hypothetical worlds are common sense: people should prefer more to less, firms should only do things that make a profit and, if the price is right, you should be prepared to give up anything you own.
Another tool used to help us understand our motivations and actions is game theory, which examines how you make choices when their outcomes are affected by the choices of others. To determine which of a number of options to go for, you need a theory about what the other person will do (and your theory needs to encompass the other person's theory about what you will do, and so on). Rational actor theory says other players in the game all want the best outcome for themselves, and that they will assume the same about you.
The most famous game in game theory is the “prisoner's dilemma”, in which you are one of a pair of criminals arrested and held in separate cells. The police make you this offer: you can inform on your partner, in which case you either get off scot free (if your partner keeps quiet), or you both get a few years in prison (if he informs on you too). Alternatively you can keep quiet, in which case you either get a few years (if your partner also keeps quiet), or you get a long sentence (if he informs on you, leading to him getting off scot free). Your partner, of course, faces exactly the same choice.
If you're a rational actor, it's an easy decision. You should inform on your partner in crime because if he keeps quiet, you go free, and if he informs on you, both of you go to prison, but the sentence will be either the same length or shorter than if you keep quiet.
Weirdly, and thankfully, this isn't what happens if you ask real people to play the prisoner's dilemma. Around the world, in most societies, most people maintain the criminals' pact of silence. The exceptions who opt to act solely in their own interests are known in economics as "free riders" – individuals who take benefits without paying costs.
The prisoner's dilemma is a theoretical tool, but there are plenty of parallel choices – and free riders – in the real world. People who are always late for appointments with others don't have to hurry or wait for others. Some use roads and hospitals without paying their taxes. There are lots of interesting reasons why most of us turn up on time and don't avoid paying taxes, even though these might be the selfish "rational" choices according to most economic models.
Crucially, rational actor theory appears more useful for predicting the actions of certain groups of people. One group who have been found to free ride more than others in repeated studies is people who have studied economics. In a study published in 1993, Robert Frank and colleagues from Cornell University, in Ithaca, New York State, tested this idea with a version of the prisoner's dilemma game. Economics students "informed on" other players 60% of the time, while those studying other subjects did so 39% of the time. Men have previously been found to be more self-interested in such tests, and more men study economics than women. However even after controlling for this sex difference, Frank found economics students were 17% more likely to take the selfish route when playing the prisoner's dilemma.