Developed by American economist Edward Chamberlin (1899-1967) and English economist Joan Robinson (1903-1983), monopolistic competition refers to competition between several firms producing an almost identical product in a market.
The demand for each good is not perfectly elastic. Monopolistic firms command brand loyalty and therefore are not price-takers. Under this form of competition, total product equals the sum of marginal cost and marginal revenue.
E H Chamberlin, A Theory of Monopolistic Competition (Cambridge, Mass., 1933)