What is monopoly deadweight loss and under what conditions does it arise?

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Monopoly deadweight loss measures the loss in total surplus that happens when a single seller with market power restricts output below the level that would be socially optimal. In a monopoly, the quantity produced is where marginal revenue equals marginal cost, but the price consumers pay ends up above marginal cost. That mismatch means there are potential trades where the value to buyers (the price they’re willing to pay) would exceed the cost of producing the good, but those trades don’t occur because output is limited. The result is a triangular area of lost surplus between the demand curve and the marginal cost curve from the monopoly quantity up to the competitive, socially efficient quantity.

This occurs because firms have market power—monopoly or other imperfect competition—not because of, say, a general gain in total surplus or underproduction when price is below cost. The other statements don’t fit the standard DWL story: it’s a loss, not a gain; price typically exceeds marginal cost, not the other way around; and while price discrimination can shift who captures surplus, it doesn’t erase the fundamental deadweight loss from underproduction under market power.