A monopolist maximizes at MR = MC, hence MC might be higher than necessary. How is this affected by a horizontal M&A?

Recall that monopolists do not produce at lowest possible MC, but rather at MR = MC. This is closely related to strategic/organizational literature that says firms care more about market power than inefficiency. Hence, MC might be higher than necessary and then MR is set based on MC, and then the monopolist operates at MR = MC that is higher than necessary. After a merger, this can go both ways: a. By decreasing number of firms may increase market power and therefore increase inefficencies, negative effect of merger (-) b. Merger might reduce the number of inefficient firms, positive effect on costs savings, merger poitive effect (+)