It has become commonplace in international development to intervene upon communities with interventions that require collective action without first gauging the communities’ perceptions regarding their ability and autonomy to engender and maintain change. Programmes and research studies employing community-based interventions often overlook important interpersonal behavioural factors that may well affect uptake and effectiveness. Social constructs such as collective efficacy, social capital, social cohesion, and social norms are important interpersonal behavioural factors and predictors of collective action, which may be needed to realise downstream health and development impacts. This doctoral thesis examined the conceptualisations of various social constructs and their theoretical underpinnings. Theoretical examinations were used to establish hypotheses regarding the underlying structure of collective efficacy (CE). A factor analytic approach was used to develop four CE measurement scales from data collected in Odisha, India and Amhara, Ethiopia to empirically test these hypotheses. The India CE scale was subsequently used to ascertain whether there were independent associations between CE-factors and the uptake and influence of a community-based water supply and sanitation intervention. Compared to controls, intervention households had higher CE factor scores for village leadership (β=0.16, 95% CI: 0.08, 0.25) and agency (β=0.08, 95% CI: 0.01, 0.15), and lower scores for social disorder, conditional on education. Prevalence of improved WASH behaviours was associated with CE factors: improved water piped on-premise was associated with village leadership (aPR=2.25, 95% CI: 1.12, 4.53); improved on-site toilets with social disorder (aPR=0.90, 95% CI: 0.84, 0.97); enclosed bathing rooms with social response (aPR=1.12, 95% CI: 1.02, 1.23); and utilisation of improved sanitation facilities by all family members with agency (aPR=1.17, 95% CI: 1.00, 1.37) and village leadership (aPR=3.86, 95% CI: 1.67, 8.97). Agency, social response, and social disorder factors were associated with nutritional outcomes. Implications for enhanced intervention design, targeting, and evaluation are discussed throughout.