Classical economical theories assume whatever information is available about products will be fully and efficiently processed by consumers, thus the increase of information amount would lead to greater diversity of decisions. However, consumers’ information processing capacity is limited. A large amount of product alternatives that requests consumers’ intensive cognitive effort will activate their social learning by switching decision strategy and relying on the information of product popularity. Thus, this experimental study attempts to discover the mixture of consumers’ rationality and sociality in online shopping. A 2*2 factorial design was conducted to tease out the effects of choice set and product popularity on individuals’ cognitive efforts and preference reversal, as well as the effects on the market structures that are aggregated from individual decisions. The experimental results demonstrate that, despite a large amount of product alternatives, participants may not always invest abundant cognitive efforts and the market is more concentrated with the presence of product popularity. Theoretical and practical implications are discussed.