A meta-analysis of the effects of measuring theory of planned behaviour constructs on behaviour within prospective studies.

Measurement reactivity effects, such as the mere measurement effect, have been proposed as a reason for behavioural changes in a number of theory of planned behaviour intervention studies. However, it is unclear whether such changes are the result of the mere measurement effect or of other artefacts of intervention study design. The aim of this study is to determine the size and direction of changes in health behaviours from baseline to follow-up in prospective studies using the theory of planned behaviour. Electronic databases were searched for the theory of planned behaviour studies which measured health behaviours at two or more time points. Change in behaviour was calculated for all studies. Sixty-six studies were included. Mean effect sizes across all studies were small and negative (d = -.03). Effect size was moderated by behaviour, behaviour type and follow-up length. Subgroup analyses showed significant decreases in socially undesirable behaviour (d = -.28), binge drinking (d = -.17), risk driving (d = -.20), sugar snack consumption (d = -.43) and sun-protective behaviour (d = -.18). Measurement of intention at baseline resulted in significant decreases in undesirable behaviour. Changes in undesirable behaviours reported in other studies may be the result of the mere measurement effect.