The argumentative theory of reasoning describes the exchange of arguments between people as the main function of reasoning. The aim of this study is to examine the differences in the accuracy and judgements of confidence in the correctness of the answer in reasoning tasks before and after group interaction in two argumentative conditions. 104 psychology students of the Faculty of Humanities and Social Sciences in Rijeka participated in the study. First, the subjects were solving the reasoning tasks individually, then in a group of three, and again individually. In the first and second individual phase, after giving a written argument of why they chose each answer, they were also asked to estimate their confidence in the correctness of given answers. There were two experimental conditions that differed only in the group phase of the study. Approximately half of the subjects in the group phase had to only read theirs and listen to the others’ answers and the arguments. The other half of the subjects was instructed to, after each of them read their answers and arguments, discuss about the solution of each task. In both experimental conditions the performance of the individuals in the second individual phase was better compared to the performance in the first individual phase with a significantly better performance of the individuals in the discussing groups condition for two out of four tasks. Furthermore, there were significantly more changes from heterogenous groups to homogeneously correct groups after the discussion than after the argument readings. There was also a higher percentage of changes from incorrect to correct answers in the discussing condition. Individuals who were less confident in the correctness of their answer in the first individual phase were more likely to change their answer after the group phase than those who were more confident. The results obtained in this experiment imply the importance of group discussion in bettering the performance in reasoning tasks. These results can be interpreted in support of the argumentative theory which highlights the role of exchange and evaluation of arguments during a discussion.