Nonetheless, there is research indicating gender differences in math vary greatly between countries and appear to be correlated to other measures of gender equity [2]. Even in countries with relatively large discrepancies, there's not a big difference. I couldn't find any meta analysis of specifically programming skill, but an analysis of one CS department didn't find gender differences due to ability [3]. An older study showed women and men scored similarly on an introductory class, but women were less likely to get an A or A+ [4]. A small study of elementary students showed that girls and boys did equally well on easy problems, but boys did better on hard problems [5].
[0]: http://www.npr.org/sections/money/2014/10/21/357629765/when-...
[1]: Compare these tables for math and computer science: https://www.nsf.gov/statistics/nsf13327/pdf/tab34.pdf and https://www.nsf.gov/statistics/nsf13327/pdf/tab33.pdf
[2]: http://psycnet.apa.org/journals/bul/136/1/103/; PDF https://www.researchgate.net/profile/Nicole_Else-Quest/publi...
[3]: http://www.pd.infn.it/~lacaprar/ProgettoScuola/Biblio/gender...
[4]: http://files.eric.ed.gov/fulltext/ED384389.pdf
[5]: http://ase.tufts.edu/DevTech/publications/Sullivan_Gender%20...