What primarily causes a change in the quantity demanded?

A change in the quantity demanded is primarily caused by a change in price. This relationship is a fundamental principle of economics, encapsulated in 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.

When prices fluctuate, consumers adjust their purchasing decisions based on the new price levels. For instance, if the price of a product falls, it becomes more affordable, leading more consumers to purchase it, thus increasing the quantity demanded. Conversely, if prices rise, some consumers may find the product too expensive and either buy less of it or stop purchasing it altogether.

Changes in consumer preferences, income, or the number of buyers can influence overall demand, but they do not directly lead to changes in the quantity demanded of a specific good at a given price. These factors might shift the entire demand curve rather than simply cause movement along the curve, which is specifically what occurs when there is a price change.