Diminishing Marginal Utility
Diminishing marginal utility as well is to be held responsible for the rise in demand for a product when its price declines. When an individual purchases a product, he swaps his money revenue with product in order to increase his satisfaction. He continues to buy goods and services as long as marginal utility of money (MUm) is lesser than marginal utility of commodity (MUC).
Given the price of a commodity, he modifies his purchase so that MUC = MUm. This plan works well under both Marshallian assumption of constant MUmand Hicksian assumption of diminishing MUm. When price falls, (MUm = Pc) < MUC. So, equilibrium state is upset. To get back his equilibrium state, i.e., MUm = PC, = MUC, he purchases more quantities of the commodity. For, when supply of a commodity rises, its MU falls and once again MUm = MUC. For this reason, demand for a product increases when its price falls