In a monopoly market structure, the firm faces a downward sloping demand curve, contrasting with the flat demand curve experienced by firms in perfect competition. This unique demand curve leads to two significant effects when the price changes: the price effect and the output effect. Understanding these effects is crucial for analyzing how monopolies generate revenue.

When a monopoly decreases its price, the price effect comes into play. This effect indicates that the firm will earn less revenue for each unit sold due to the lower price. Conversely, the output effect suggests that the firm will sell more units at this reduced price, potentially increasing total revenue. These two effects are inherently oppositional; when the price effect decreases revenue, the output effect increases it, and vice versa when the price is raised.

Importantly, the relationship between price and marginal revenue in a monopoly is distinct. The marginal revenue (MR) for a monopolist is always less than the price (P) of the good. This occurs because, to sell additional units, the monopolist must lower the price not only on the additional units sold but also on all previous units. Therefore, the equation can be expressed as:

$$ P > MR $$

This principle is also applicable to monopolistic competition, where the revenue dynamics mirror those of a monopoly. Both market structures exhibit similar behaviors regarding average revenue and marginal revenue, reinforcing the understanding of how firms operate under different competitive conditions.

In summary, the interplay between price effect and output effect is fundamental in determining a monopolist's revenue, with the key takeaway being that marginal revenue is consistently lower than the price charged for the product. This concept will be further illustrated through examples and graphical representations in subsequent discussions.