Theoretical assumptions are often used in the fields of economics and other social sciences.
Many problems in this field have a large number of complex variables that affect and interact with each other. In order to facilitate research, people generally believe that some secondary variables are in a special state, so that they can be ignored or easy to calculate. In this way, complex problems can be simplified, and at the same time, the essence of the problem can be grasped for in-depth research.
After a theory is formed on the basis of a series of assumptions, subsequent researchers can further improve the research and interpretation of the matter by gradually relaxing the assumptions based on previous studies in order to make theoretical progress.
Example of classical theoretical hypothesis: definition of perfectly competitive market
western economics It is believed that the market is the most favorable weapon for economic regulation supply So as to adjust the mechanism of economy, people market structure It is divided into several forms. Among them, the perfectly competitive market is theoretically the most efficient one market structure , is the ideal market state. such market structure It has characteristics that cannot be realized in reality, and only through these characteristics can we determine its impact on economic efficiency Contribution of. Therefore, economists assume the following: The market of this structure is assumed to have a large number of manufacturers (unlimited), and the output provided by manufacturers is relative to market size It only accounts for a small share (infinitesimal), there is no difference between products, and manufacturers are free to enter and exit. At the same time, in a perfectly competitive market, it is assumed that the information of manufacturers and consumers in the market is completely symmetrical, so the price is absolutely transparent, and each manufacturer faces a fixed market price, so the marginal revenue is equal to Average income Both are equal to the market price. Through the constraints of the above theoretical assumptions, we can draw the following conclusions: under the condition that price equals marginal cost, in the short term, perfect competition The manufacturer can obtain Excess profit , access Normal profit Or loss. However, in the loss state, only when the price is higher than the lowest position of the average variable cost, the manufacturer will supply a positive quantity of goods; Otherwise, the business will be stopped. Therefore, perfect competition The lowest average variable cost of the manufacturer is also called Closed outlets Or close point. The marginal cost curve above the closing point is the manufacturer's Short term supply curve 。 In the long term, manufacturers adjust various production factors To minimize the cost per output. At the same time, the manufacturer shall Marginal revenue Equal to marginal cost Principle of profit maximization Determine output. Due to the entry or exit of manufacturers in the industry for a long time Long term equilibrium If the price is equal to the marginal cost average cost 。 from industry From the perspective of, the entry or exit of manufacturers may affect their costs. If the manufacturer enters industry If the cost of the manufacturer is constant, increasing or decreasing, the industry can be defined as the industry type with constant cost, increasing cost or decreasing cost Industry supply curve They are parallel to the quantity axis, slant to the upper right, and slant to the lower right. According to the principle that the price is equal to the marginal cost and the price is equal to the lowest average cost A perfectly competitive market is considered to be efficient in terms of production quantity and technology use. 
