Open defecation is a public health problem worldwide. NGOs in developing countries use various approaches to increase latrine coverage, but for little-understood reasons, some of the population does not adopt latrine construction. The objective of our research was to uncover which of the factors predicting latrine construction are relevant to the last nonowners of latrines, termed laggards in the diffusion of innovations theory. In a cross-sectional study, quantitative face-to-face interviews were conducted in households in rural Malawi (N = 824) to assess the behavioral determinants of latrine construction, mental health, and leadership. Around 14% of the households interviewed did not own a latrine. Study results suggest that nonowners have limited economic resources and perceive that latrine construction is expensive, that it is difficult to find money for latrine construction, and that it needs a lot of time and effort. The last nonowners of latrines live in smaller groups than latrine owners, communicate less with others about latrine construction, and are less influenced by the opinion of their leaders. They consist, in particular, of socially vulnerable households, are younger, are less educated, often have more impaired mental health, feel more vulnerable to contracting diseases, are less aware of the latrine construction of others in the village, feel less personally obliged to construct their own latrines, and are less confident in their ability to rebuild latrines damaged by flooding. The study confirmed that the assumptions of the diffusion of innovation theory are useful in combination with the risks, attitudes, norms, abilities, and self-regulation behavior change approach for developing evidence-based behavior change strategies in developing countries.