A number of papers have suggested that pro-social behaviour in the workplace may be sensitive to the institutional environment, but there is little empirical research that attempts to test this directly using data on worker behaviour. This is the aim of this paper. We show that individuals in the non-profit sector are significantly more likely to do unpaid overtime than those in the for-profit sector. However, we find no evidence of adjustment along either the extensive or intensive margins when individuals change sectors. The results of our analysis therefore point to selection and we find supporting evidence that individuals do self-select on the basis of their propensity to donate labour.