Suppose they have a written contract, or even an unwritten contract, what enforces the contract for colluding firms or crooks in jail?

假如他们签了一份书面协议,或干脆口头的,他们怎么强制履行呢

耶鲁公开课 - 博弈论课程节选

He probably could have a written contract about tidying up.

你们可以写个书面协议

耶鲁公开课 - 博弈论课程节选

Yeah, so maybe even among your roommates, maybe you don't need a contract because you can manage to achieve the same ends, by the fact that you're going to be interacting with the same person, over and over again during your time at Yale.

没错,就算你和室友没有书面协议,你们一样能达到预期的目的,因为在耶鲁大学上学期间,你们总要碰面

耶鲁公开课 - 博弈论课程节选

The reason the mafia thrives in countries where it's hard to write legal contracts-- let's say some new parts of the former Soviet Union or some parts of Africa the reason the mafia thrives in those environments, is that it substitutes for the law and enforces both legal and illegal contracts.

黑恶势力在那些,书面协议不受保护国家不断壮大,比如前苏联新成员国 或者非洲,为什么黑恶势力在这些地方发展壮大,因为这是法律强制力的补充,它维系所有合同,不管是不是合法的

耶鲁公开课 - 博弈论课程节选