A rapidly growing literature characterises reasoning as a social phenomenon. On the one hand, Mercier and Sperber (2011) argue that the main function of reasoning is for speakers to persuade group members, e.g., of a given belief or decision. On the other hand, Tomasello (2014) emphasises that reasoning is also critical in cooperative contexts, when groups need to find a good and justified solution benefiting all group members, rather than members competing for individual argumentative success. The literature on children's reasoning, however, focuses on their use of justifications in either conflictual/competitive or cooperative contexts without a systematic comparison of how young children's arguments change across contexts. Also, investigating how children judge arguments and use these judgements as informative input to further tasks would deepen our knowledge about their reasoning abilities. In my dissertation, I therefore investigated with whom children prefer to reason and how they reason with same-age peers as partners in cooperative and competitive contexts. In the first study, I investigated whether children who can choose between cooperative partners penalise candidates that accept poor reasons. 5- and 7-year-old children were presented with two candidates for solving a cooperative problem. In the experimental condition, one acceded to a good reason, the other to a poor reason. In the control condition, each agreed to a different good reason. Crucially, in both conditions, both arrived at a wrong conclusion. Results suggest that 7-year-olds, and 5-year-olds to a lesser degree, preferred the partner who followed the good reason in the experimental condition, but they showed no preference in the control condition. Thus, young children prefer partners who respond to good reasons over partners who comply more easily. This provides evidence that children evaluate reasons even in absence of one objectively correct informant, and use this evaluation to judge cooperation partners. In the second study, I used a more interactive paradigm and investigated how 5- and 7-year-old children reason with peer partners in cooperative and competitive contexts. Peer dyads were asked to place animals in various places in a zoo, which included unusual items such as an alarm clock. One child received training, playing the game with the experimenter before playing with his/her naive peer and learned a set of critical arguments (e.g., 'Bears hibernate so they need the alarm clock to wake up when winter is over'). When the trained child played the game with a naive peer, reproducing these arguments would help both of them in the cooperative condition but worsen the trained child's chances of winning in the competitive condition. The results suggest that 7-year-olds, but not 5-year-olds withheld the trained arguments more often in the competitive condition than in the cooperative condition. Thus, this study suggests that children's reasoning is less biased or less 'inhibited' in cooperative settings in which a joint decision benefits both parties. In the third study, I investigated whether combining cooperative and competitive contexts (namely, cooperating as a team against competitor) would facilitate children's reasoning further. Similar to the second study, 5- and 7-year-olds were asked to cooperate in a task (e.g., decorating a zoo). In the main condition, children were competing against another group, whereas in the control condition, they were not. The prediction was that children would show more productive reasoning and richer discussions in the main condition. However, this prediction did not bear out. Out-group competition did not trigger richer reasoning between partners. I discuss this result in addition to a set of theoretical remarks. As a whole, this set of studies suggests that starting in their sixth year, children start to identify and prefer to reason with people who submit to reason. They display less biased reasoning in cooperative contexts, in which the goal is to reach good joint decisions that benefit all group members. And out-group competition itself does not have a specific positive influence on children's cooperative reasoning.