This is a myth that has been around for a while, likely it is the result of societies where women are seen as the "lesser sex". Like the U.S. circa, well, almost all of our history as a country, throughout most of which, there were a lot less women in more math oriented careers than men (like engineers or physicists, or, mathematicians). The shame is it seems that this myth became self perpetuating. That is, despite the fact that no real evidence emerged to show that men are any better at math than women*, some evidence suggested that girls who were aware of the negative stereotype (that they are supposedly worse at math than boys) tended to do worse on standardized math tests than girls who were unaware of the stereotype. A new study (really a compilation of other studies allowing for a meta-analysis) further bears this out. It shows that differences between the sexes in math ability are more likely a reflection of the country (or local environment) in which the subjects live, rather than a reflection of some hidden math prowess that is conferred by having a Y chromosome. The study found that, in countries where women were treated as being lesser than men, and where there were few positive women role models (in terms of math and science), girls performed worse than boys on the math tests, BUT, in countries where men and women are on more equal footing, and there are more prominent female role models, the girls actually did as well or BETTER than the boys. You go, girls!
*sure, there were a couple studies showing that boys did better on standardized math tests than girls, but then there's a ton of data that shows that girls tend to get better grades than boys, including grades in math and science classes.