The Process by Which Relative Autonomous Motivation Affects Intentional Behavior: Comparing Effects Across Dieting and Exercise Behaviors

A motivational model integrating self-determination theory and the theory of planned behavior was tested in two samples for exercise and dieting behavior respectively. Relative autonomous motivation from selfdetermination theory was hypothesized to predict intentions to exercise or diet via the mediation of attitudes and perceived behavioral control (PBC) from the theory of planned behavior. It was also expected that attitudes and PBC would predict actual levels of exercise and dieting behavior via the mediation of intentions. Relations in the proposed model were expected to be invariant across the behaviors. Two samples of participants (N = 511) completed measures of the autonomous motives, attitudes, subjective norms, PBC, and intentions with respect to exercise and dieting behavior. Four weeks later, participants self-reported their behavior. Structural equation models supported the replicability of the proposed model in both behaviors. Findings supported the majority of the hypothesized effects in the proposed model across the two health behaviors. However, four effects were significantly different across the two behaviors: the effect of autonomous motives on intentions, subjective norms andPBC on intentions, and intentions on behavior. Findings extend knowledge of the processes by which psychological antecedents from the theories affect health behaviors integral to the maintenance of energy balance.