In Norway's case, they're mandatory - a law put in place several years ago requires companies to have 40% of the board directors to be women. A large number of companies appear to have just ignored that, putting them in rather an awkward position now that the Norwegian government is actually saying that they will apply the law they've been saying they'll apply.
Here's the quote:
""the fact that diversity is a value in itself, that it creates wealth... From my time in the business world, I saw how board members were picked: they come from the same small circle of people. They go hunting and fishing together. They're buddies."
Norway being Norway, that quote is from a 52 year old man in their Conservative party, Ansgar Gabrielsen.
But it really hits the nail on the head - if they are only selecting from a limited pool - those who will be the fishing buddies of the current limited pool - then you are restricting the quality of your boards. Given that Norway has had equal education for years, if the quality of the female candidates for board director isn't up to scratch, then that will be the fault of the companies themselves for not promoting them to positions to gain the experience needed. The Norwegian government had given them ample time to do so.