The present study explored factors that promote change in discussants' explanations for a social phenomenon through a highly ill-defined problem solving discussion. Specifically, causal model 1 (engagement in conflicting and⁄or cooperative discourse promotes cognitive change) and causal model 2 (questioning by other triggers one's explanatory activity which results in his⁄her cognitive change) were mainly examined. Forty-three college students were divided into 10 groups, which consist of 4-5 members each. Each group was asked to construct a hypothetical causal explanation, which explains the causes of Japanese teenager's impulsive aggression. All discussions were videotaped and coded in terms of conversational function with a coding schema developed by Tomida & Maruno (2000). Frequencies of coded utterances that each discussant generated during discussion were utilized as main variables. As results, although the model 2 was supported, the model 1 was partially supported. That is, while cooperative utterances facilitated cognitive change, conflicting utterances had no such an effect. Additionally, examining relationships among frequencies of utterances, we found cooperative utterances elicit explanatory activity. Considering a fact that explanatory activity clearly led changes in explanations, we speculated that the two causal models can be integrated.