Doing something that hurts everyone to limit the amount you’re hurt.
The Prisoner’s Dilemma is a thought experiment wherein two people who’ve committed a crime are brought into separate interrogation rooms and given a choice wherein they can incriminate their old partner, giving that person a longer sentence, or shorten the severity of their own punishment.
Outcomes:
If A rats on B and B rats on A then
they are both given 2 years of prison.
If A rats on B and B does not incriminate A then
A walks free
B goes to prison for 3 years
If A does not incriminate B and B rats on A then
A goes to prison for 3 years
B walks free
If A does not incriminate B and B does not incriminate A then
they both walk free
In the dilemma, a rational self-interested agent would increment their partner. It gives them the least punishment, even if it guarantees that their partner ends up in jail for at least 1 year.
This has all sorts of real-world practical implications, particularly in a military sense. See related notes.