New Zealand that has a low unemployment is still at around 3 %.
There is also social efficency where MEC + MPC = MSB+MEB. Firms will definitely not always go by the rule and the marginal external benefit or cost is usually ignored and not paid for.
Thus in theory the efficency can be achieved at times but individual firms might not want to have it as a goal because of for want of profit that is not guaranteed in some cases.
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