at least in the USA it's clear that girls are steered away from STEM curricula by a variety of mechansims, primarily social. Girls aren't "supposed" to like Maths, and so they don't. Girls that show an interest or aptitude are subtly or not so subtly told that this is inappropriate, that they're weird, that they will be unpopular, and so on.
In fact until they're told they should be bad at it, girls generally show higher aptitude for maths than boys of comparable age. -- Charles
