Final answer:
Individual firms in an oligopoly have an incentive to produce more than the quantity that maximizes joint profits due to the temptation of higher profits. If some firms produce more, the market price will fall and firms may end up with zero economic profits.
Step-by-step explanation:
In an oligopoly, each individual firm has an incentive to produce more than the quantity that maximizes their joint profits due to the temptation to increase their own profit margin. While oligopolists recognize that they would benefit by acting collectively like a monopoly, they may choose to produce slightly more to earn slightly higher profit with the expectation that other firms will hold down their production and keep prices high. However, if some firms give in to this temptation and start producing more, the market price will fall, and all firms may end up earning zero economic profits.