diffusion of treatments
a situation in which research participants adopt a different intervention from the one they were assigned because they believe the different intervention is more effective. When diffusion of treatments occurs, the internal validity of a study is called into question, as it would be difficult to attribute a specific outcome to a specific intervention. For example, participants in a weight-reduction program may be assigned to a low-carbohydrate intervention, a low-fat intervention, a low-calorie intervention, or a control group that only receives educational material on losing weight. If some participants in the control condition found out that those in one of the other conditions were losing more weight and they adopted that intervention, there would be diffusion of treatments.