**Zero Income Elasticity of Demand**

The income elasticity of demand measures the responsiveness of the quantity demanded of a good or service to a change in income. It is calculated as the percentage change in quantity demanded divided by the percentage change in income.

Zero income elasticity of demand means that the quantity purchased remains constant regardless of changes in income. In other words, there is no relationship between income and the demand for the product.

There are several reasons why the quantity purchased may remain constant despite changes in income.

**1. Necessity Goods:** Some goods are considered necessities, meaning that they are essential for basic living and survival. These goods tend to have a low income elasticity of demand because people will continue to purchase them regardless of changes in income. Examples of necessity goods include basic food items, utilities, and healthcare.

**2. Saturation:** In some cases, consumers may already have enough of a certain product and therefore do not feel the need to purchase more, even if their income increases. For example, if a consumer already owns a car, an increase in income may not lead to an increase in the quantity of cars purchased.

**3. Substitutability:** If there are readily available substitutes for a particular product, consumers may switch to those substitutes if their income decreases. However, if their income increases, they may not increase their demand for the original product. This can result in a zero income elasticity of demand.

**4. Habitual Consumption:** Some goods or services are purchased out of habit or routine, regardless of changes in income. For example, a person may continue to purchase their daily cup of coffee even if their income increases.

**5. Price Changes:** The income elasticity of demand measures the relationship between income and quantity demanded, assuming that all other factors remain constant. However, if there are simultaneous changes in both income and price, the income elasticity of demand may be affected. For example, if the price of a good increases at the same time as income increases, the quantity purchased may not change significantly.

In conclusion, when the quantity purchased remains constant irrespective of changes in income, it is referred to as zero income elasticity of demand. This can be due to various factors such as the nature of the good, saturation in the market, availability of substitutes, habitual consumption, or simultaneous changes in price and income.