enter image description here

The question I'm dealing with is:

Suppose A plays bf, Which of B's strategies would lead to an outcome that is not pareto efficient?

The answer is apparently ei as $(bf,ei)=(0,5)$, but I don't understand why. If this answer is correct, could you please explain why?

My understanding of pareto efficiency is that by moving from $(x,y)$, we cannot make any player better off without making the other worse off. If this is the case, then $(x,y)$ is pareto efficient.

$(x,y)$ would be pareto inefficient if there exists a movement away from $(x,y)$, where a player can be made better off without the other being made worse off.

From $(bf,ei)=(0,5)$, we can move to:

$(4,4), (5,0),(2,2)$

I don't see how we can move to any of these without making B worse off, I think that $(bf,ei)$ represents a pareto efficient outcome.

Any chance you could offer some advice?


  • 2
    $\begingroup$ If A would play $(af)$ while B maintained $(ei)$ the outcome would be (2,6), which Pareto dominates (0,5). $\endgroup$
    – Giskard
    Apr 21, 2015 at 22:03
  • $\begingroup$ Ah, thank you. I was only considering the outcomes from the subgame initiated after B plays e. If you want to rewrite your comment as an answer, I'd be happy to accept it. $\endgroup$
    – Five σ
    Apr 21, 2015 at 22:19
  • 1
    $\begingroup$ Thank you, but I am content with your answer. $\endgroup$
    – Giskard
    Apr 22, 2015 at 5:26

1 Answer 1


Thanks to denesp, I realised that I was only considering outcomes resulting from the following subgame:

enter image description here

If you consider all outcomes within the extensive form, then it is clear that $(2,6)$ pareto dominates $(0,5)$.


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