6.5 Model Assumptions: Monopolistic Competition

A monopolistically competitive market has features that represent a cross between a perfectly competitive market and a monopolistic market (hence the name). The following are some of the main assumptions of the model:

1. Many, many firms produce in a monopolistically competitive industry. This assumption is similar to that found in a model of perfect competition.
2. Each firm produces a product that is differentiated (i.e., different in character) from all other products produced by the other firms in the industry. Thus one firm might produce a red toothpaste with a spearmint taste, and another might produce a white toothpaste with a wintergreen taste. This assumption is similar to a monopoly market that produces a unique (or highly differentiated) product.

3. The differentiated products are imperfectly substitutable in consumption. This means that if the price of one good were to rise, some consumers would switch their purchases to another product within the industry. From the perspective of a firm in the industry, it would face a downward-sloping demand curve for its product, but the position of the demand curve would depend on the characteristics and prices of the other substitutable products produced by other firms. This assumption is intermediate between the perfectly competitive assumption in which goods are perfectly substitutable and the assumption in a monopoly market in which no substitution is possible.

   Consumer demand for differentiated products is sometimes described using two distinct approaches: the love-of-variety approach and the ideal variety approach. The love-of-variety approach assumes that each consumer has a demand for multiple varieties of a product over time. A good example of this would be restaurant meals. Most consumers who eat out frequently will also switch between restaurants, one day eating at a Chinese restaurant, another day at a Mexican restaurant, and so on. If all consumers share the same love of variety, then the aggregate market will sustain demand for many varieties of goods simultaneously. If a utility function is specified that incorporates a love of variety, then the well-being of any consumer is greater the larger the number of varieties of goods available. Thus the consumers would prefer to have twenty varieties to choose from rather than ten.

   The ideal variety approach assumes that each product consists of a collection of different characteristics. For example, each automobile has a different color, interior and exterior design, engine features, and so on. Each consumer is assumed to have different preferences over these characteristics. Since the final product consists of a composite of these characteristics, the consumer chooses a product closest to his or her ideal variety subject to the price of the good. In the aggregate, as long as consumers have different ideal varieties, the market will sustain multiple firms selling similar products. Therefore, depending on the type of consumer demand for the market, one can describe the monopolistic competition model as having consumers with heterogeneous demand (ideal variety) or homogeneous demand (love of variety).

4. There is free entry and exit of firms in response to profits in the industry. Thus firms making positive economic profits act as a signal to others to open up similar firms producing similar products. If firms are losing money (making negative economic profits), then, one by one, firms will drop out of the industry. Entry or exit affects the aggregate supply of the product in the market and forces economic profit to zero for each firm in the industry in the long run. (Note that the long run is defined as the period of time necessary to drive the economic profit to zero.) This assumption is identical to the free entry and exit assumption in a perfectly competitive market.
5. There are economies of scale in production (internal to the firm). This is incorporated as a downward-sloping average cost curve. If average costs fall when firm output increases, it means that the per-unit cost falls with an increase in the scale of production. Since monopoly markets can arise when there are large fixed costs in production and since fixed costs result in declining average costs, the assumption of economies of scale is similar to a monopoly market.

These main assumptions of the monopolistically competitive market show that the market is intermediate between a purely competitive market and a purely monopolistic market. The analysis of trade proceeds using a standard depiction of equilibrium in a monopoly market. However, the results are reinterpreted in light of these assumptions. Also, it is worth mentioning that this model is a partial equilibrium model since there is only one industry described and there is no interaction across markets based on an aggregate resource constraint.