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