Contract law provides a more prosaic example of the same logic. One of the basic problems of contracting over time is the danger of ex post opportunism. 92 When quid is exchanged simultaneously for quo, neither party is particularly exposed. On the other hand, if one party confers a benefit on the other party now in the expectation of a future benefit, the counterparty always faces the temptation to take the money and run. This temptation is particularly strong when the disappointed promisee has no means of retaliating against the breaching promisor. Seen in these terms, ex post opportunism is really a problem of invulnerability. Not surprisingly, in situations where a promisor is vulnerable to retaliation, cooperative outcomes generally
prevail.