Gendered choices of STEM subjects for matriculation are not driven by prior differences in mathematical achievement

•Gender streaming among STEM fields appears already in secondary school.•Girls are under-represented in physics, IT and advanced mathematics.•This pattern is not driven by gender differences in prior achievement in numeracy.•Socio-economic disadvantage has a greater adverse effect on boys than on girls.•There is significantly less gender streaming among STEM fields in all-girl schools.