The aim of this paper is to uncover the cognitive biases that attack potential customers in user reviews and to determine their influence on e-shop popularity ratings. In order to accomplish the objective, a survey was conducted among the TOP 100 e-shops on the Czech market, from which were selected 70 e-shops to identify the most frequently occurring cognitive biases. In the first step, the heuristic method of observing e-shop websites was used. In the second step, a chi-square two-sample test was used to obtain results due to the nominal nature of the data under study. It was found that in terms of the user interface, the most emerging biases are the bandwagon effect, apophenia, authority bias and social proof. Then, in the case of examining the reviews themselves, it was found that availability bias, story bias, processing difficulty effect, negativity effect and authority bias were determined to be the most likely to influence potential customers. Some of these biases were also found to affect the popularity ratings of the e-shop, which marketing managers should pay attention to because of the link between popularity and loyalty.