AbstractCompared choices among solutions of problem by individuals andgroups from two populations: 251 university students, aged 18 to 25 years, and 364 trade school students, aged 15 to 18 years. All subjects gave first their solution of Maiers horsetrading problem. Discussing groups were composed of one individual holding the correct answer while other members held each a different wrong solution. In one of the further individual conditions, single subjects were provided with all solutions without comment, and in the other, together with a brief summary of supportive arguments. As predicted, in the population of university students, the proportion of correct answers increases in all experimental conditions (p<.05). No significant difference is observed between individual and group choices. Group members defending the correct answer are more certain of their solution, talk more and are perceived as more confident than supporters of wrong solutions. In groups of trade school students, the reverse is true, and the proportion of correct answers decreases following both discussion (p<.01) and exposure to all solutions (p<.02). In this population, performance improves only when single subjects are provided with written arguments in favour of various solutions (p<.01). Results are analysed in terms of low solution verifiability depending on task, situational, and population factors.