The Scandinavian countries rank high, probably the highest, when it comes to gender equality. They also rank high when it comes to having a gender segregated labour market. You might find more women in STEM in a third world country that has less gender equality than in a Scandinavian country. One possible explanation is that these countries are affluent while countries with more gender inequality tend to be less affluent. If you don't have a social safety net, and maybe even have to rely on finding a husband to provide for you and all the uncertainty that goes with that, if you don't find a job for yourself that is lucrative enough, STEM might be a good choice. But if you live in a country with both a lot of gender equality and income equality? Then money doesn't factor into it that much anymore...

I tried to Google for the articles I've read about this but I didn't manage to find them.