For what reason are production possibilities frontiers usually bowed outward?

Question

Quizplus · Accepted Answer

Production possibilities frontiers are usually bowed outward because of increasing opportunity cost. This means that as production of one good increases, the opportunity cost of producing additional units of that good increases in terms of the amount of the other good that must be given up. This is because resources are not perfectly adaptable to the production of both goods, so as more and more resources are allocated to one good, the marginal return from additional units of the other good decreases. Therefore, the opportunity cost of producing additional units of the first good increases, causing the production possibilities frontier to be bowed outward.

For What Reason Are Production Possibilities Frontiers Usually Bowed Outward