One of my favorite parts of The Dark Knight was the skillfully arranged Prisoners' Dilemma situation presented by the two ferries.
A quick re-cap. The Joker has been managed to force Gotham authorities to load two ferries - one with citizens, the other with criminals. While in the water we see that the Joker has rigged each ferry with explosives and given the detonators to each boat. The detonators, we are told, are linked to the other boat's explosives. Each boat has until midnight to detonate the other ship's ordnance or BOTH ferries will be destroyed.
Each ferry has to make a choice: kill or be killed - confess or stay silent. And the clock is ticking.
*SPOILER* Of course, neither ferry is able to carry through with the deed and both detonators are left un-activated. (Ed's note: I wonder if Nolan chose the ferry full of convicts to be first in remaining silent as they would be more familiar with this type of situation whereas the other ferry seemed quicker to
judge - to confess.)
However, according to the Dilemma, each ferry must still pay a price - there are no free rides once you are forced into this position. What price is it that the people on each ferry have to pay? Perhaps the price is living with the uncomfortable knowledge that you considered confessing - killing - to save your own life.
What is critical to the point of The Dark Knight is the role that this dilemma plays in the strengthening of community (indeed, human) bonds that maintain the denizens of Gotham.
The likelihood of defection in a population may be reduced by the experience of cooperation in earlier games allowing trust to build up. Hence self-sacrificing behaviour may, in some instances, strengthen the moral fibre of a group. If the group is small the positive behaviour is more likely to feed back in a mutually affirming way, encouraging individuals within that group to continue to cooperate.
See also: Nash Equilibrium
UPDATE: This guy does a way better job of explaining this than I.