The demand curve slopes downwards due to the following reasons
(1) Substitution effect: When the price of a commodity falls, it becomes relatively cheaper than other substitute commodities. This induces the consumer to substitute the commodity whose price has fallen for other commodities, which have now become relatively expensive. As a result of this substitution effect, the quantity demanded of the commodity, whose price has fallen, rises.
(2) Income effect: When the price of a commodity falls, the consumer can buy more quantity of the commodity with his given income, as a result of a fall in the price of the commodity, consumer's real income or purchasing power increases. This increase induces the consumer to buy more of that commodity. This is called income effect.
(3) Number of consumers: When price of a commodity is relatively high, only few consumers can afford to buy it, And when its price falls, more numbers of consumers would start buying it because some of those who previously could not afford to buy may now afford to buy it, Thus, when the price of a commodity falls, the number of its consumers increases and this also tends to raise the market demand for the commodity.
(4) various uses of a commodity
(5) law of diminishing marginal utility
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