The size of a customer base can shift the demand curve. This may occur when there is an overall increase in population, such as the Baby Boomer generation, or when the size of your usual base increases due to customers climbing the class scale. Market size can especially cause a demand curve to shift if the product or service in question is a "need" and not just a "want." For example, a necessity such as soap may experience heightened demand during population increases simply because more people are buying the non-discretionary item.