What is the main cause of a change in quantity demanded?

The main cause of a change in quantity demanded is indeed a change in price. This relationship is fundamental in economics and is often illustrated by the law of demand, which states that, all else being equal, as the price of a good or service decreases, the quantity demanded increases, and conversely, as the price increases, the quantity demanded decreases. This occurs because consumers typically react to price changes by adjusting the amount they are willing and able to purchase.

Price changes directly affect consumers' purchasing decisions; when prices drop, consumers perceive goods as more affordable, leading them to buy more. Conversely, if prices rise, goods may become less accessible, resulting in a decrease in quantity demanded. This relationship showcases the basic principle of how market dynamics operate, reflecting changes in consumer behavior in response to price fluctuations.

On the other hand, factors such as substitution, changes in revenue, or the income effect can influence demand but are not direct causes of changes in quantity demanded. They may shift the entire demand curve or affect consumer behavior more broadly but do not specifically trigger changes in quantity demanded in the same way that price changes do.