Behavioural Short Read

The Allais Paradox: Explained

The Nobel Prize-winning economist, Maurice Allais, posed this famous paradox in a 1953 Econometrica article. It led to the discovery of one of the most significant notions in behavioural economics today: loss aversion

It is a Tuesday afternoon. George, exhausted from a tough senior league match, trudges into Tudors. He looks up at the display and gasps: all the paninis are gone expect two – a turkey and a chicken and pesto! George ponders which one he would most enjoy and snatches the turkey. Just as he is about to pass it to Mr Tudor, he notices a freshly prepared batch of tomato and mozzarella paninis being put on the display. After a brief hesitation, he changes his mind. He puts down the turkey and picks up the chicken and pesto.

In this imagined scenario, George, a half-witted D Blocker who has not yet been enlightened by economic theory, acted irrationally. He changed his decision based on information that was irrelevant: the fact that there was a third type of panini left that he did not pick.

Assuming that people are rational when given a choice between A and B, they will pick the option that they regard as having the most utility: the one that is more valuable to them for the money that they spend on it. If an option C (in our case, the tomato and mozzarella panini) presents itself, it should not incite a switch from A to B or vice versa, because each option’s utility had not changed.

Historically, most economists believed that the general populous was sharper than George when it came to preferences with uncertain outcomes, i.e. the vast majority of choices that we make. Until recently, the reigning theory of decision making was the expected utility theory, developed by Daniel Bernoulli in 1738. It states that people make choices that maximise their expected utility – utility of an option multiplied by the probability of it occurring. The conclusion was that people act rationally at least most of the time. This is significant, because if people act rationally, then markets act rationally. If markets act rationally, then the state should have minimal involvement in the economy, which was for decades the advice most economists gave to politicians.

In 1953, Maurice Allais, a French economist, presented one of the most substantial arguments against expected utility theory to date. It became known as the Allais Paradox and is outlined below for you to try on yourself.


Stage 1. Would you rather get…

A) $5 million for sure

Or

B) An 89% chance of winning $5 million
A 10% chance of winning $15 million
A 1% chance of winning nothing

The majority of people pick A: the certainty of earning $5 million over a slim chance of getting even more. Now consider the second stage of the paradox.

Stage 2. Would you rather get…

C) An 11% chance of winning $5 million
An 89% chance of winning nothing

Or

D) A 10% chance of winning $15 million
A 90% chance of winning nothing


This time, almost everyone votes for D. Understandably, the more substantial payout outweighs the slightly higher risk of ending up with nothing. On the face of it, in both stages people act rationally; they sensibly judge the payout of their decisions against the odds involved, trying to maximise their expected utility. In actuality, in first choosing A and then D, they violate the expected utility theory. They act irrationally. Consider the following proof:


Let u stand for utility

In stage 1, we determined that u(A) > u(B)

∴ 1u(5 million) > 0.89u(5 million) + 0.1u(15 million) + 0.01u(0)

0.11u(5 million) > 0.1u(15 million) + 0.01u(0)

In stage 2, we determined that u(D) > u(C)

∴ 0.1u(15 million) + 0.9u(0) > 0.11u(5 million) + 0.89u(0)

0.11u(5 million) < 0.1u(15 million) + 0.01u(0)

This is the reverse of the conclusion we arrived at earlier.


Similarly to George, our choices were altered by seemingly irrelevant information. Stage 2 is the same as stage 1 except for an 89% reduction in the chance of winning $5 million in both options. The expected utility of 5 million dollars and 15 million dollars had not changed, yet our choices were incoherent.

In the 1970s, Amos Tversky and Daniel Kahneman, two psychologists who throughout their careers had upended many perceptions about how the human mind functions, developed a theory that explained our behaviour when presented with the Allais Paradox. They realised that we are more sensitive to the difference between 100% chance of winning at least $5 million (A) and 99% (B) than we are to the difference between 11% (C) and 10% (D). That is why we opt for A and then D. We value complete certainty disproportionately.

“In human decision making, losses loom larger than gains.” – Amos Tversky

What Tversky and Kahneman found was that people think of different outcomes in terms of relative changes rather than absolute changes. If they choose B and end up with nothing, they regard this outcome as a loss of $5 million despite the fact that there are no richer or poorer than they were before. This happens because A offered complete certainty in winning. If they choose D and end up with nothing, they do not regard it as a loss because the odds presented by C were not much better. People are more sensitive to losses than to gains, making them risk averse.

This discovery, sparked by the Allais Paradox, helped Kahneman win a Nobel Prize in Economics in 2002. Even more significantly, it contributed to the foundation of the new and exciting field of behavioural economics.

Further reading

I would recommend reading What is rationality in Economics? by Seb Carpanini. It draws a distinction in the meaning of rationality in psychology and economics.

I would also strongly encourage reading The Undoing Project by Michael Lewis. It examines the complicated relationship between Amos Tversky and Daniel Kahneman while exploring the brilliant ideas about the human mind that they had developed together.

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