"It also goes, in my mind, some way to explaining why these countries with some of the best legislation around women’s equality ( according to UN) do not see greater numbers of women in STEM."
How so? Many countries which have much worse legislations and attitudes towards women's equality have gender ratios in STEM that are near to 50:50.
My theory is that school teachers, who are predominantly female, tend to respond more approvingly to girls and rate their abilities more highly. This means that girls tend to rate themselves more highly in subjectively marked subjects (arts, languages, humanities), where the teacher's judgement is the most important factor in how a student is rated. In contrast, boys rate themselves more highly in objectively marked subjects (STEM subjects) where the marking criteria is not much affected by the teacher's judgement. This then leads girls to choose non-STEM degrees and boys to choose STEM degrees.
The exceptions are:
a. Girls from non-white minority backgrounds, who favour objective subjects due to discrimination from white female teachers (this explains why most British women studying STEM PhDs in the UK are non-white).
b. Boys who attended all-boys schools, who did not experience gendered discrimination by female teachers and therefore are much more likely to choose to study non-STEM subjects than boys at mixed schools.
Just check the numbers. ALL of the numbers agree with my theory.