I think we have to move forward with the assumption that girls can do it, because it hurts nobody, and see how it works out. The only alternative is to effectively give up on girls, "You didn't catch up fast enough!", and stop providing them with the support they need to overcome implicit bias, minority status, and so on.
The National Science Foundation has spent $130 million to encourage more women in STEM
http://www.universityaffairs.ca/exa...-gender-gap-in-the-stem-disciplines.aspx , based on the belief that disparities are inequities. I think that money could have been better spent on the NSF's primary mission of funding scientific research.
Everyone agrees that the distribution of math ability in the two sexes overlaps -- there are lots of girls who are better than the average boy. Why should the belief that the male distribution is centered slightly to the right of the female distribution have such a discouraging effect on females? Accepting equal ability as dogma often leads to the conclusion that we know fewer women are in STEM because society is stacked against them. I think holding this belief could be more discouraging to a girl than the belief that there is a small difference in the distribution of ability.