While public health in most countries of the world is better now than it has ever been, a huge burden of preventable disease still remains: our behaviour does not seem to match our knowledge. In this chapter we focus on the psychological mismatch between the environments in which we evolved and in which we now live. We show that most current public health problems can be explained by maladaptive behaviour in the context of massive environmental changes, most having occurred since the Industrial Revolution 150 years ago. We show how almost all of our major public health problems are associated with motivated behaviour, usually because we over- or under- use evolutionarily novel technologies. Hence, while we can trace suboptimal health to a lack of fit between our evolved motives and our current environment, understanding of these motivational drivers can help us to modify behaviour, environments, and technologies such that they generate healthier outcomes. We give an example of how ancient motives can be harnessed for the benefit of public health in the case of handwashing with soap – a novel health protective technology.