Before explorers stumbled across Australia, Europeans thought all swans were white. And the more white swans people saw, the more they believed this was true. It wasn't until the late 17th century when Dutch sea captain Willem de Vlamingh sighted a large flock of black swans on what is now the Swan River in Western Australia that this belief was overturned.
Women in mathematics, like black swans, are unexpected on the face of it.
Maths has widely been accepted as a male discipline for the longest time. As a female mathematician I regularly see first-hand the imbalance of numbers in the discipline. Can we then conclude that girls must not like maths as much as boys? Or perhaps that on the whole they are just not as competent at the subject?
It's not that women don't like maths or aren't capable, it's just that society by design has stopped women going into the discipline and that's why there are fewer females.
A counter example to a generalisation can have the power to unhinge even the most established of beliefs — just as Vlamingh's black swans did to the white swan theory.
Along with my STEM (science, technology, engineering and maths) sisters, I am the metaphorical black swan. Centuries ago we were few and far between, today a larger, but still relatively small number of us swim around in the STEM pond.