The income and substitution effects account for

A down-ward slope is represented on the demand curve graph to show when the substitution effect begins. The reason for the down-ward slope is when prices increase for specific goods and services and consumer's disposable income doesn't increase respectively to compensate, the demand for these goods decrease. The substitution effect begins because consumers start to shop for similar goods to satisfy their demand for a lower price.

For example, the price of brand-named carbonated beverages increase from 50 cents to one dollar per 20 ounce bottle. When consumers are accustomed to paying only 50 cents per bottle, they will begin purchasing generic brand beverages for a cheaper price. This causes the demand for the brand-name to decrease.