A large part of a person's cognitive and interpretive process is formed unconsciously. Although he may make conscious decisions at times, while he is apparently making conscious choices, his thinking and decision-making are deeply influenced by the unconscious process that dominates his mind. It is at this stage that cognitive biases occur. The main question of the present research is how cognitive bias affects the judge's judgment? To find a more precise answer, the subject of the article is limited to the effect of cognitive bias in establishing causation. In this article, we will see that the uncertainty of the legal system in the issue of causation and the reference of legal doctrine to custom to determine the cause have caused the judge to use various tools such as expert theory to establish custom and to make decisions on the issue of attributing responsibility by assuming himself as an average and ordinary member of society, which brings into play the judges' intuitive judgments and tendencies. It is in these circumstances that cognitive biases find a suitable ground for their emergence and impact. Also, there is no way to completely prevent the occurrence and eliminate the negative effects of cognitive biases because, on the one hand, judges will not be able to formulate a verdict without biases and, on the other hand, these matters, like the process of dreaming, occur unconsciously and cannot be controlled. However, awareness of these biases is necessary. This awareness can be provided in the form of various laws or in the form of basic training for judges before or during their training. The present research examines these issues in a descriptive-analytical manner using direct findings from psychology in law.