Math and Gender

The AP reports that the gender gap in math performance declines or disappears in countries where greater gender equity exist. The study, conducted by a Northwestern University professor, shows that U.S. is right in the middle of the pack in terms of gender equity and math scores. Gender equity was determined by such factors as the World Economic Forum's gender equity index and the math was based on the PISA exam.

According to the AP article, the report does not look at the reasons behind the findings, but I'd be curious to know what others think about this. Why does the gap still persist? How can we bridge it?