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Using meta-analysis, the impact of a number of manifestations of social influence (important others, family, class leaders, coexercisers, social cohesion, and task cohesion) on exercise behaviors (adherence and compliance), cognitions (intentions and efficacy), and affect (satisfaction and attitude) was examined. The results showed that social influence generally has a small to moderate positive effect (i.e., effect size ES from .20 to .50). However, four moderate to large effect sizes (i.e., ES from .50 to .80) were found: family support and attitudes about exercise, task cohesion and adherence behavior, important others and attitudes about exercise, and family support and compliance behavior.
Carron et al. (Fri,) studied this question.