Do policies favouring low tuition fees have an effect on enrolment in scientific and quantitative fields of study? We estimate the causal effect of a change in the cost of higher education and separate the compositional effects, measured by the differences in skills between students attracted to university because of the lower costs, from the structural (or behavioural) effects measured by the changes in university stream induced by this same reduction. Our results indicate that any expansion of the university system induced by new students attracted by a reduction in costs would contribute to changing the distribution of the fields of study chosen and in particular to reducing the proportion of STEM students, but would also reduce the propensity of students already enrolled to choose the STEM field.