Washing hands with soap at the right times – primarily after contact with faeces, but also before handling food or feeding an infant – can significantly reduce the incidence of childhood infectious disease. Here, we present empirical results which substantiate a recent claim that washing hands can be the consequence of different kinds of psychological causes. Such causes can be divided into three kinds of control over behaviour: automatic or habitual responses, motivated or goal-driven behaviour to satisfy needs, and cognitive causes which reflect conscious concerns. Empirical results are based on 3-h-long structured observations of hand-washing behaviour in 802 nationally representative Kenyan households with children under five, and structured interviews with the primary female caretaker in these households, collected in March 2007. Factor analysis of questionnaire responses identified three psychological factors which are also significant predictors of observed hand-washing behaviour: having the habit of hand-washing at particular junctures during the day, the motivated need for personal or household cleanliness, and a lack of cognitive concern about the cost of soap use. These factors each represent a different kind of psychological cause. A perceived link between clean hands and sexual attractiveness also appeared in the factor analysis, but was not a determinant of actual behaviour. We also report evidence that those who express concern about the cost of soap use are those with relatively few economic resources. We suggest that those developing hygiene promotion programmes should consider the possible existence of multiple types of strategies for increasing hand-washing behaviour.