Table 1. The trade-off between targeting amenable or resistant individuals with behavioural interventions.

Subset of population targeted by the intervention	Effect of the intervention
	Direct effect	Indirect effect
Amenable individuals	Targeting those most likely to change behaviour maximizes the direct effect of the intervention	Targeting those most likely to change behaviour means relying on the indirect effect of the intervention to influence those least likely to change. For a given magnitude of direct effect, targeting those most likely to change minimizes the indirect effect of the intervention and, by extension, behavioural spillover
Resistant individuals	Targeting those least likely to change behaviour minimizes the direct effect of the intervention	For a given magnitude of direct effect, targeting those least likely to change behaviour maximizes the indirect effect of the intervention because those most likely to change will be affected indirectly but were not directly targeted
Randomly selected individuals	The size of the direct effect will be intermediate between the effects of targeting either amenable or resistant individuals	The size of the indirect effect will be intermediate between the effects of targeting either amenable or resistant individuals