Recent policy in England has suggested that educational outcomes will be raised if schools specialise in particular subjects. In contrast, calls for the reform of 16-19 education have suggested that these outcomes will be improved if students become less specialised in their studies. At present, there is a limited evidence base from which to judge these arguments. In particular, we do not know the extent to which students' achievements in 16-19 education are higher when they choose subjects which play to their perceived strengths. We also do not know whether students are more likely to choose to study subjects taught by more effective departments. That is, outcomes may be affected by the relative strengths of students or departments in circumstances where there is freedom to choose. In this paper we provide evidence of the existence and strength of these relationships. This evidence suggests that reducing the scope within schools for specialisation or competition will reduce average student attainment and these effects ought to be taken into account when evaluating alternative curriculum policies. (Contains 4 tables.)