This paper compares psychosocial and cognitive models of socioeconomic variation in participation in screening for colorectal cancer. The psychosocial model suggests that factors such as higher stress and lower social support explain, in part, why people from lower socioeconomic status (SES) environments are less likely to participate in screening. The cognitive model suggests that beliefs about cancer risk and screening will play an important part in differential participation. In practice both sets of factors may contribute to explaining socioeconomic differentials. The data for these analyses are drawn from a randomised controlled trial of colorectal cancer screening (the UK Flexible Sigmoidoscopy Trial). The participants are from the Scottish centre, where recruitment was stratified to generate a socioeconomically diverse sample. The dependent variable was interest in attending screening. A questionnaire covering demographic status, psychosocial and cognitive factors as well as interest in screening was sent to 10,650 adults. The results showed the predicted SES gradient in interest. There were also SES differences in both psychosocial and cognitive variables. A series of logistic regression models were used to test potential mediators of the association between SES and interest in attending screening by successively including psychosocial factors, cognitive factors, and then both, in the equation. Only the inclusion of the cognitive variables significantly reduced the variation associated with SES, providing better support for the cognitive than the psychosocial model.