One of the cornerstones of the R system for statistical computing is the multitude of contributed packages making an extremely broad range of statistical techniques and other quantitative methods freely available. This study investigates which factors are the crucial determinants responsible for the participation of the package authors in the R project. For this purpose a survey was conducted among R package authors, collecting data on different types of participation in the R project, three psychometric scales (hybrid forms of motivation, work design characteristics, and values), as well as various specie-demographic factors. These data are analyzed using item response theory and generalized linear models, showing that the most important determinants for participation are a hybrid form of motivation and the knowledge characteristics of the work design. Other factors are found to have less impact or influence only specific aspects of participation. (authors' abstract) / Series: Research Report Series / Department of Statistics and Mathematics