But, why do we assume the demand curves of individuals slope down? In the case of Market Demand, we argued that, when price falls, ceteris paribus, new buyers would enter the market, and some current buyers would increase their quantity demanded as the relative price of the good in question changed. But, new buyers cannot be an influence on an individual's demand curve; the individual in question is the buyer. How de we justify the assumption that individual demand curves have a negative slope? If they do not, then we may not be able to add them to get the market demand.