The influence of social 'factor on joint group thinking activity in triads is discussed. The interrelations of group performance, interpersonal connections' character and offered task structure are shown. Effectiveness of group performance is dependent of social set of interpersonal connections as well as of the type of task. The extent of fixed set influence varies according to thinking task complexity. Complicated task (mathematical riddle) lessens differential influence of group interrelations (e. i. whether relations are positive or negative). Positive social set stimulates group members less when high communication level is necessary for task solving.