In the case of necessities, an increase in income will have a positive influence on the demand for the product, but not to the same extent as with luxury goods. Should incomes decrease, the demand for necessities will not be affected that much.
For instance, if your income increases, you might consider buying those products that you normally do not buy, eating out more often, going to the movies or spoiling yourself with a pedicure. When times are tough, those will be the first goods that you will drop. However, if your income decreases, you will not cut back on your spending on necessities that much – you still have to eat.