Final answer:
The demand curve can slope up when there is an increase in income or when a substitute product becomes more expensive.
Step-by-step explanation:
The demand curve typically slopes downwards because as the price of a product increases, consumers are willing to buy less of it. However, there are situations where the demand curve can slope upwards.
One example is when an increase in income leads to higher demand for a product. In this case, a rightward shift in the demand curve occurs, showing that at any given price, the quantity demanded is higher. This can be seen in Figure 3.8 of the utility maximization framework, where an increase in income causes the demand curve to shift upwards.
Another example is when a substitute product becomes more expensive. If the price of a substitute good increases, consumers may be willing to buy less of that good and more of the original good, causing the demand curve for the original good to slope upwards.