If the income elasticity of demand for a good is negative, then the good must be an inferior good.

Income Elasticity

A measure of how much the demand for a good or service changes in response to a change in the consumer's income.

Inferior Good

A type of good for which demand decreases as the income of the consumer increases, opposite to normal goods.

Negative

In the context of economics, denotes a situation or indicator that reflects a decrease, deficit, or detrimental condition.