In BG and the USSR there used to be quotas and also less choice over what to study, so about 50% of STEM students were women. When I studied in BG the quotas we

In BG and the USSR there used to be quotas and also less choice over what to study, so about 50% of STEM students were women. When I studied in BG the quotas were long gone and 20 out of 160 students in our engineering specialty were women. Girls greatly outperform boys in Maths at school, but tend to choose non-STEM degrees nowadays. I think the introduction of choice in the system greatly impacted the outcome, because many desirable fields were not commonly available (i.e. Socialism planning does not allow for many lawyers and accountants, language study was quite limited, things like International Relations was effectively only for offspring of high-rank party officials).