Consider the problem of open defecation, a widespread problem in South Asia that has recently received a lot of attention.
Is it a structural challenge, i.e., that functioning toilets aren't available to the population engaging in the behaviour?
Or is it a behavioural challenge, i.e., that people with access to clean, functioning toilets continue to engage in open defecation?
When the problem is structural, supply chain and systems design will be much more effective than a behavioural science lens. But if the problem is behavioural, studying it from a behavioural science and social norms perspective would be more helpful.